are expanded in steps like `COPY`
- `ARG` and `ENV` are both set as environment variables in `RUN`

???

- An ARG before a FROM applies only to FROM lines
- An ARG after a from line applies to all the following steps, until another FROM line

---

# Dockerfile

- `LABEL` sets an image label
- `EXPOSE` metadata on ports the container will likely use
- `USER` changes the default user for commands
- `WORKDIR` sets the default directory, similar to `cd`
- `VOLUME` breaks things
- `HEALTHCHECK` tells you when th