[CI] Use proper docker builds with caching in github actions

[joerunde]

Currently our build setup is a very simple docker build command: https://github.com/vllm-project/vllm-spyre/blob/main/.github/workflows/test-spyre.yml#L10-L11. We could instead import the docker build action, and set up caching so that we don't have to rebuild the full image on every test run

[ckadner]

I would like to take a stab at this

[github-actions]

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