fix: Normalize docker tags

[Raffy23]

This PR fixes and issue in the CI pipeline if a GitHub user has any uppercase letters in their name (e.g. Raffy23). This is done by transforming all TAGS to lowercase to ensure that all tags are lowercase.

The Problem occurs when using the following docker-build action:

     - name: Docker Build
        id: docker_build
        uses: keptn/gh-automation/.github/actions/docker-build
        with:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          TAGS: |
            ghcr.io/${{ github.repository_owner }}/${{ env.IMAGE }}:${{ env.VERSION }}
          BUILD_ARGS: |
            version=${{ env.VERSION }}

the action will fail with

buildx failed with: error: invalid tag "ghcr.io/Raffy23/automatic-octo-telegram:0.0.2-dev": repository name must be lowercase

A successful run of the GitHub Action can be seen here: https://github.com/Raffy23/automatic-octo-telegram/actions/runs/1944722398.