- My editor converted some of these to spaces when I changed the lines
in the previous commit. We should be consistent, so I made all of them
into spaces.
- I suggested this for the contents of
[Linuxbrew/docker](https://github.com/Linuxbrew/docker) in
https://github.com/Linuxbrew/docker/issues/75. People agreed, and
Shaun asked me to do the same here.
- This adds a step to CI to lint the Dockerfile, via
[hadolint](https://github.com/hadolint/hadolint), on Ubuntu.
- The linting errors it surfaced on this Dockerfile were:
```
Dockerfile:4 DL3008 Pin versions in apt get install. Instead of `apt-get install <package>` use `apt-get install <package>=<version>`
Dockerfile:30 DL3020 Use COPY instead of ADD for files and folders
Dockerfile:32 DL3003 Use WORKDIR to switch to a directory
```
- [DL3008](https://github.com/hadolint/hadolint/wiki/DL3008) - pinning
versions in `apt-get install` - is at odds with what we recommend in the
normal Homebrew on Linux dependency install instructions. We don't
want the dependency management of having to check each of these
Dockerfiles periodically for the latest version numbers of packages
and have to update them. So I've disabled this lint.
- [DL3003](https://github.com/hadolint/hadolint/wiki/DL3003) - use
WORKDIR to `cd` - is disabled in this case due to [review
comments](https://github.com/Homebrew/brew/pull/7433/files#r415098255).
