Build the docker image from a ciflow/docker label

[huydhn]

• Build the tutorials image from a ciflow/docker label instead of on every PR, because fork PRs can't get the AWS token needed to push to ECR.
• The labeler adds the label when .ci/docker changes.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/tutorials/3977

• 📄 Preview Python docs built from this PR

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As of commit ff5d4c0 with merge base c80a434 ():
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[huydhn]

Not needed for now: #3976 builds the image on its ciflow/docs run when it is missing, and carries the docker-build.yml trigger fixes. Keeping the branch in case we want a separate ciflow/docker label later.

[huydhn]

Reopened: #3976 is stacked on this again, and relies on it to build the image for PRs that change .ci/docker.