Enforce using a lowercase value for Image Name

[Th0masL]

It seems like ghcr.io does not support uppercase in the image name.

Since the current CI workflow use the value of ${{ github.repository }}, anyone that has at least one uppercase letter in their username (like me) will fail to push to ghcr.io with the following error:

Error: buildx failed with: ERROR: invalid tag "ghcr.io/Th0masL/podsync:latest": repository name must be lowercase

See error here : https://github.com/Th0masL/podsync/actions/runs/5499113405/jobs/10021072765#step:5:116

More explanations here : https://github.com/orgs/community/discussions/25768

[mxpv]

This seems like can be fixed in the forked repository. There is no need to carry this here?

[Th0masL]

It would be more convenient to fix this here instead of in the fork directory, as it would prevent the fork to be easily rebased with your main branch