Add Docker image build and publish workflows for prefect-gcp and prefect-azure

[devin-ai-integration]

Closes #12367

Adds Docker image build and publish workflows for prefect-gcp and prefect-azure, following the existing pattern established by prefect-aws.

What's included

For each integration (prefect-gcp, prefect-azure):

• Dockerfile (src/integrations/prefect-{gcp,azure}/Dockerfile) — installs pinned versions of prefect and the integration package on top of the base prefecthq/prefect image, with a smoke test
• Build + publish workflow (.github/workflows/prefect-{gcp,azure}-docker-images.yaml) — supports workflow_dispatch and workflow_call, builds multi-arch (amd64/arm64) images across Python 3.10–3.14, pushes to DockerHub with the same tagging scheme as prefect-aws
• PR test build workflow (.github/workflows/prefect-{gcp,azure}-docker-test.yaml) — triggered on PRs touching the Dockerfile or workflow files, does a test build on amd64 only and verifies the installed version matches expectations

Important review items

• The publish workflows reference secrets.PREFECT_GCP_DOCKERHUB_TOKEN and secrets.PREFECT_AZURE_DOCKERHUB_TOKEN — these secrets need to be created in the repo settings before the workflows can push images.
• The workflows push to prefecthq/prefect-gcp and prefecthq/prefect-azure on DockerHub — these repositories need to exist.
• All files are a mechanical adaptation of the prefect-aws equivalents — worth a quick scan to confirm no stale aws references were left behind.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

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Link to Devin run: https://app.devin.ai/sessions/095467a78415450391f6d7f363764805
Requested by: @desertaxle

[devin-ai-integration]

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