Make unit tests build faster by basing the image off base-php instead of the full app image

[megahirt]

Description

Re-architect the test-php image to be built off the base-php image for faster building and execution of unit tests. This should get unit tests running in under 2 minutes without all the UI building happening in the full app image (not necessary for PHP unit tests)

Also, use a docker layer cache action.

Informal testing:

• before this PR: unit tests complete in 4 minutes
• this PR, without layer caching: unit tests complete in 2 minutes
• this PR, layer caching first run (create cache): 2.5 minutes
• this PR, with existing cache: 1.5 minutes 🥇

Testing on your branch

• Unit tests run correctly on my local machine
• Unit tests run correctly on CI

Checklist

• I have performed a self-review of my own code
• I have reviewed the title/description of this PR which will be used as the squashed PR commit message
• I have commented my code, particularly in hard-to-understand areas

qa.languageforge.org testing

Reviewers: add/replace your name below and check the box to sign-off/attest the feature works as expected on qa.languageforge.org

• Reviewer1 (YYYY-MM-DD HH:MM)
• Reviewer2 (YYYY-MM-DD HH:MM)

[longrunningprocess]

let's pair on the this, I see some stuff that needs to be pulled out.