Use commit SHA for manual deploys when branch name starts with 'v'

[theseanything]

There's a bug in the deployment system which causes manual deployments of branch names being with 'v' to use the branch name to tag the images, instead of the commit SHA for the HEAD of the branch. This prevents subsequent commits in the same branch from being deployed as the image continues to have the same tag.

This is caused by our rather blunt logic to determine if a gitRef is a tag.

govuk-infrastructure/.github/workflows/build-and-push-multiarch-image.yml

Line 78 in 28a9dfd

type=raw,priority=500,value=${{ inputs.gitRef }},enable=${{ startsWith(inputs.gitRef, 'v') }}

We can't test github.ref to see if it starts with refs/tags/v as workflow dispatch calls (i.e. manual deploys) will always set it to main. We'd like to still support manual deploys of version tags.

We probably could just run git describe --exact-match --tags which will fail if the repository isn’t checked out to a tag.