- 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).

workflows/tests: ensure taps are up-to-date

ENV/std: port superenv SDK changes

shims/super/cc: relax restrictions with -Xclang

ENV/super: always set HOMEBREW_SDKROOT for Xcode builds

sh: fix custom shell prompt for zsh

ruby.sh: allow running on older Ruby versions

bintray: fix style

workflows/tests: link old gettext.

This is failing `brew doctor` on GitHub Actions.

dev-cmd/audit: fix false openssl@1.1 versioned formula audit

bintray: fix package creation

formula_installer: fix build_from_source? call.

build(deps-dev): bump html-proofer from 3.15.2 to 3.15.3 in /docs

This should check the local instance variable rather than the global
value as it varies for dependencies and requested formulae.

Bumps [html-proofer](https://github.com/gjtorikian/html-proofer) from 3.15.2 to 3.15.3.
- [Release notes](https://github.com/gjtorikian/html-proofer/releases)
- [Commits](https://github.com/gjtorikian/html-proofer/compare/v3.15.2...v3.15.3

)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

workflows/tests: allow missing system Linuxbrew.

If it's not there: setup Linuxbrew but reuse the checkout.

pr-upload: Fix a bug introduced by PR #7406

Fix the error:
Error: No such file or directory @ rb_sysopen - bottle

uses_from_macos: Add gzip rsync to the white list

pr-upload: Add argument root-url

pull: fix head_revision definition.

Fixes #7405.

/usr/bin/gzip and /usr/bin/rsync are provided by macOS.

The argument root-url is needed for third-party taps.

ARGV: Deprecate ARGV.force_bottle?

diagnostic: whitelist /bin and /sbin for *-config files