We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
CentOS 6 image doesn't build with clingo on Dockerhub
### Steps to reproduce

Has to do with failure on centos:6

```
Step 17/19 : RUN spack spec hdf5+mpi
 ---> Running in 8335d48ff53f
==> Warning: Spack will not check SSL certificates. You need to update your Python to enable certificate verification.
==> Warning: Spack will not check SSL certificates. You need to update your Python to enable certificate verification.
==> Warning: the original concretizer is currently being used.
        Upgrade to "clingo" at your earliest convenience. The original concretizer will be removed from Spack starting at v0.18.0
==> Error: cannot bootstrap the "clingo" Python module from spec "clingo-bootstrap@spack+python %gcc target=x86_64"
Input spec
--------------------------------
hdf5+mpi

Concretized
--------------------------------
==> Bootstrapping clingo from pre-built binaries
The command 'docker-shell spack spec hdf5+mpi' returned a non-zero code: 3
```

---

So it bootstraps *during* concretization?

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `lib/spack/spack/schema/container.py`
Content:
```
1 # Copyright 2013-2021 Lawrence Livermore National Security, LLC and other
2 # Spack Project Developers. See the top-level COPYRIGHT file for details.
3 #
4 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
5 """Schema for the 'container' subsection of Spack environments."""
6 
7 _stages_from_dockerhub = {
8     'type': 'object',
9     'additionalProperties': False,
10     'properties': {
11         'os': {
12             'type': 'string',
13             'enum': ['ubuntu:18.04',
14                      'ubuntu:16.04',
15                      'centos:7',
16                      'centos:6']
17         },
18         'spack': {
19             'type': 'string',
20         },
21     },
22     'required': ['os', 'spack']
23 }
24 
25 _custom_stages = {
26     'type': 'object',
27     'additionalProperties': False,
28     'properties': {
29         'build': {'type': 'string'},
30         'final': {'type': 'string'}
31     },
32     'required': ['build', 'final']
33 }
34 
35 #: List of packages for the schema below
36 _list_of_packages = {
37     'type': 'array',
38     'items': {
39         'type': 'string'
40     }
41 }
42 
43 #: Schema for the container attribute included in Spack environments
44 container_schema = {
45     'type': 'object',
46     'additionalProperties': False,
47     'properties': {
48         # The recipe formats that are currently supported by the command
49         'format': {
50             'type': 'string',
51             'enum': ['docker', 'singularity']
52         },
53         # Describes the base image to start from and the version
54         # of Spack to be used
55         'images': {'anyOf': [_stages_from_dockerhub, _custom_stages]},
56         # Whether or not to strip installed binaries
57         'strip': {
58             'type': 'boolean',
59             'default': True
60         },
61         # Additional system packages that are needed at runtime
62         'os_packages': {
63             'type': 'object',
64             'properties': {
65                 'command': {'type': 'string', 'enum': ['apt', 'yum']},
66                 'update': {'type': 'boolean'},
67                 'build': _list_of_packages,
68                 'final': _list_of_packages
69             },
70             'additionalProperties': False
71         },
72         # Add labels to the image
73         'labels': {
74             'type': 'object',
75         },
76         # Add a custom extra section at the bottom of a stage
77         'extra_instructions': {
78             'type': 'object',
79             'additionalProperties': False,
80             'properties': {
81                 'build': {'type': 'string'},
82                 'final': {'type': 'string'}
83             }
84         },
85         # Reserved for properties that are specific to each format
86         'singularity': {
87             'type': 'object',
88             'additionalProperties': False,
89             'default': {},
90             'properties': {
91                 'runscript': {'type': 'string'},
92                 'startscript': {'type': 'string'},
93                 'test': {'type': 'string'},
94                 'help': {'type': 'string'}
95             }
96         },
97         'docker': {
98             'type': 'object',
99             'additionalProperties': False,
100             'default': {},
101         }
102     }
103 }
104 
105 properties = {'container': container_schema}
106
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/lib/spack/spack/schema/container.py b/lib/spack/spack/schema/container.py
--- a/lib/spack/spack/schema/container.py
+++ b/lib/spack/spack/schema/container.py
@@ -12,8 +12,7 @@
             'type': 'string',
             'enum': ['ubuntu:18.04',
                      'ubuntu:16.04',
-                     'centos:7',
-                     'centos:6']
+                     'centos:7']
         },
         'spack': {
             'type': 'string',

{"golden_diff": "diff --git a/lib/spack/spack/schema/container.py b/lib/spack/spack/schema/container.py\n--- a/lib/spack/spack/schema/container.py\n+++ b/lib/spack/spack/schema/container.py\n@@ -12,8 +12,7 @@\n             'type': 'string',\n             'enum': ['ubuntu:18.04',\n                      'ubuntu:16.04',\n-                     'centos:7',\n-                     'centos:6']\n+                     'centos:7']\n         },\n         'spack': {\n             'type': 'string',\n", "issue": "CentOS 6 image doesn't build with clingo on Dockerhub\n### Steps to reproduce\r\n\r\nHas to do with failure on centos:6\r\n\r\n```\r\nStep 17/19 : RUN spack spec hdf5+mpi\r\n ---> Running in 8335d48ff53f\r\n==> Warning: Spack will not check SSL certificates. You need to update your Python to enable certificate verification.\r\n==> Warning: Spack will not check SSL certificates. You need to update your Python to enable certificate verification.\r\n==> Warning: the original concretizer is currently being used.\r\n        Upgrade to \"clingo\" at your earliest convenience. The original concretizer will be removed from Spack starting at v0.18.0\r\n==> Error: cannot bootstrap the \"clingo\" Python module from spec \"clingo-bootstrap@spack+python %gcc target=x86_64\"\r\nInput spec\r\n--------------------------------\r\nhdf5+mpi\r\n\r\nConcretized\r\n--------------------------------\r\n==> Bootstrapping clingo from pre-built binaries\r\nThe command 'docker-shell spack spec hdf5+mpi' returned a non-zero code: 3\r\n```\r\n\r\n---\r\n\r\nSo it bootstraps *during* concretization?\n", "before_files": [{"content": "# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other\n# Spack Project Developers. See the top-level COPYRIGHT file for details.\n#\n# SPDX-License-Identifier: (Apache-2.0 OR MIT)\n\"\"\"Schema for the 'container' subsection of Spack environments.\"\"\"\n\n_stages_from_dockerhub = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        'os': {\n            'type': 'string',\n            'enum': ['ubuntu:18.04',\n                     'ubuntu:16.04',\n                     'centos:7',\n                     'centos:6']\n        },\n        'spack': {\n            'type': 'string',\n        },\n    },\n    'required': ['os', 'spack']\n}\n\n_custom_stages = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        'build': {'type': 'string'},\n        'final': {'type': 'string'}\n    },\n    'required': ['build', 'final']\n}\n\n#: List of packages for the schema below\n_list_of_packages = {\n    'type': 'array',\n    'items': {\n        'type': 'string'\n    }\n}\n\n#: Schema for the container attribute included in Spack environments\ncontainer_schema = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        # The recipe formats that are currently supported by the command\n        'format': {\n            'type': 'string',\n            'enum': ['docker', 'singularity']\n        },\n        # Describes the base image to start from and the version\n        # of Spack to be used\n        'images': {'anyOf': [_stages_from_dockerhub, _custom_stages]},\n        # Whether or not to strip installed binaries\n        'strip': {\n            'type': 'boolean',\n            'default': True\n        },\n        # Additional system packages that are needed at runtime\n        'os_packages': {\n            'type': 'object',\n            'properties': {\n                'command': {'type': 'string', 'enum': ['apt', 'yum']},\n                'update': {'type': 'boolean'},\n                'build': _list_of_packages,\n                'final': _list_of_packages\n            },\n            'additionalProperties': False\n        },\n        # Add labels to the image\n        'labels': {\n            'type': 'object',\n        },\n        # Add a custom extra section at the bottom of a stage\n        'extra_instructions': {\n            'type': 'object',\n            'additionalProperties': False,\n            'properties': {\n                'build': {'type': 'string'},\n                'final': {'type': 'string'}\n            }\n        },\n        # Reserved for properties that are specific to each format\n        'singularity': {\n            'type': 'object',\n            'additionalProperties': False,\n            'default': {},\n            'properties': {\n                'runscript': {'type': 'string'},\n                'startscript': {'type': 'string'},\n                'test': {'type': 'string'},\n                'help': {'type': 'string'}\n            }\n        },\n        'docker': {\n            'type': 'object',\n            'additionalProperties': False,\n            'default': {},\n        }\n    }\n}\n\nproperties = {'container': container_schema}\n", "path": "lib/spack/spack/schema/container.py"}], "after_files": [{"content": "# Copyright 2013-2021 Lawrence Livermore National Security, LLC and other\n# Spack Project Developers. See the top-level COPYRIGHT file for details.\n#\n# SPDX-License-Identifier: (Apache-2.0 OR MIT)\n\"\"\"Schema for the 'container' subsection of Spack environments.\"\"\"\n\n_stages_from_dockerhub = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        'os': {\n            'type': 'string',\n            'enum': ['ubuntu:18.04',\n                     'ubuntu:16.04',\n                     'centos:7']\n        },\n        'spack': {\n            'type': 'string',\n        },\n    },\n    'required': ['os', 'spack']\n}\n\n_custom_stages = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        'build': {'type': 'string'},\n        'final': {'type': 'string'}\n    },\n    'required': ['build', 'final']\n}\n\n#: List of packages for the schema below\n_list_of_packages = {\n    'type': 'array',\n    'items': {\n        'type': 'string'\n    }\n}\n\n#: Schema for the container attribute included in Spack environments\ncontainer_schema = {\n    'type': 'object',\n    'additionalProperties': False,\n    'properties': {\n        # The recipe formats that are currently supported by the command\n        'format': {\n            'type': 'string',\n            'enum': ['docker', 'singularity']\n        },\n        # Describes the base image to start from and the version\n        # of Spack to be used\n        'images': {'anyOf': [_stages_from_dockerhub, _custom_stages]},\n        # Whether or not to strip installed binaries\n        'strip': {\n            'type': 'boolean',\n            'default': True\n        },\n        # Additional system packages that are needed at runtime\n        'os_packages': {\n            'type': 'object',\n            'properties': {\n                'command': {'type': 'string', 'enum': ['apt', 'yum']},\n                'update': {'type': 'boolean'},\n                'build': _list_of_packages,\n                'final': _list_of_packages\n            },\n            'additionalProperties': False\n        },\n        # Add labels to the image\n        'labels': {\n            'type': 'object',\n        },\n        # Add a custom extra section at the bottom of a stage\n        'extra_instructions': {\n            'type': 'object',\n            'additionalProperties': False,\n            'properties': {\n                'build': {'type': 'string'},\n                'final': {'type': 'string'}\n            }\n        },\n        # Reserved for properties that are specific to each format\n        'singularity': {\n            'type': 'object',\n            'additionalProperties': False,\n            'default': {},\n            'properties': {\n                'runscript': {'type': 'string'},\n                'startscript': {'type': 'string'},\n                'test': {'type': 'string'},\n                'help': {'type': 'string'}\n            }\n        },\n        'docker': {\n            'type': 'object',\n            'additionalProperties': False,\n            'default': {},\n        }\n    }\n}\n\nproperties = {'container': container_schema}\n", "path": "lib/spack/spack/schema/container.py"}]}