Minimise Docker builds in CI

[khvn26]

The following Docker builds are unneccessary:

For E2E:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 55 to 56 in 1e75ae9

  	uses: ./.github/actions/local-e2e-tests
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 84 to 85 in 1e75ae9

  	uses: ./.github/actions/local-e2e-tests
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 113 to 114 in 1e75ae9

  	uses: ./.github/actions/local-e2e-tests
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 142 to 143 in 1e75ae9

  	uses: ./.github/actions/local-e2e-tests
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 270 to 271 in 1e75ae9

  	uses: depot/build-push-action@v1
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 293 to 294 in 1e75ae9

  	uses: depot/build-push-action@v1
  	with:

• flagsmith/.github/workflows/platform-pull-request.yml

  Lines 317 to 318 in 1e75ae9

  	uses: depot/build-push-action@v1
  	with:

• flagsmith/.github/workflows/platform-test-merge-to-main.yml

  Lines 32 to 33 in 1e75ae9

  	id: run-local-api
  	uses: ./.github/actions/run-local-api

For Uffizzi:

• flagsmith/.github/workflows/uffizzi-build.yml

  Lines 40 to 41 in 1e75ae9

  	uses: depot/build-push-action@v1
  	with:

All of the above are building the same couple of images. We should build them once per CI run, push to ephemeral registry, and pass the reference around.

[matthewelwell]

I think we should tie into this work, the addition of e.g. Docker Scout into our CI pipeline.

https://github.com/docker/scout-action