CI/Docker: Fix hadolint errors in [build.Dockerfile], do not allow the hadolint job to fail

What

A hadolinting error was introduced to build.Dockerfile in !11890 (merged). This MR fixes the error.

It slipped through the CI because the hadolint was set to allow_failure: true, which is removed by this MR.

Why

I'm not sure why we had allow_failure: true on this job before, but I'm fairly sure it's an error (try following it in git blame if you're curious).

We should extend hadolint to all dockerfiles in the project, but I found so many warnings that I thought it best to deal with it in a separate MR.

How

Not much to say.

Manually testing the MR

Check out the Hadolint job in this MR's pipelines.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR