Test docker image builds on PRs

[mvandenburgh]

This will make it so a "test build" of docker images (i.e. they will get built but not published to GHCR) gets performed on any PR that makes changes to the images/ directory. This will help catch image build failures before a merge to main happens.

[scottwittenburg]

Makes sense to me, and seems like the part where it builds on PRs is working. If there's anything wrong with the push condition, we won't know until it's run on main.

[mvandenburgh]

Looks like it is working as expected on main right now -

Run docker/build-push-action@2eb1c1961a95fc15694676618e422e8ba1d63825
  with:
    context: ./images/gh-gl-sync
    file: ./images/gh-gl-sync/Dockerfile
    push: true
    tags: ghcr.io/spack/ci-bridge:0.0.[3](https://github.com/spack/spack-infrastructure/actions/runs/6644910333/job/18055087620#step:6:3)8
    platforms: linux/amd6[4](https://github.com/spack/spack-infrastructure/actions/runs/6644910333/job/18055087620#step:6:4),linux/arm[6](https://github.com/spack/spack-infrastructure/actions/runs/6644910333/job/18055087620#step:6:6)4
    load: false
    no-cache: false
    pull: false
    github-token: ***
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Docker info