Improve the resilience of the Backstop Docker image build

[techanvil]

Feature Description

As discussed here, #10788 (comment), we should update the Backstop Dockerfile to help avoid our Chromium version going out of sync again. We should also take the opportunity to further refine the Dockerfile, and to avoid the need to force the build on each run as discussed in the linked comment.

---

Do not alter or remove anything below. The following sections will be managed by moderators only.

Acceptance criteria

The Backstop Dockerfile should be updated as follows:

• The version of Chromium should be installed in a way that doesn't depend on the version in the APT repository, so as to avoid the image build failing unexpectedly with future repository updates.
• The image tag should include a version to ensure it's fully rebuilt when changes are made, helping to avoid hard to spot caching issues (e.g. the apt-get update stage is cached but could result in different build outputs).
• The RUN commands should be consolidated where it makes sense to do so.

Implementation Brief

Test Coverage

QA Brief

Changelog entry

[benbowler]

FYI #10486 exists to use a built version of the image which will help keep this consistent between runs as the dependencies will be set at build time and only updated when the image is rebuilt and published.

[techanvil]

Thanks @benbowler, that's a good point. I think the AC for this issue remain valid even taking that into account, but it's useful to highlight the link between the issues.