fix(docker-publish): handle Uppercased chars repository

[Nico385412]

Hello 👋

I noticed the workflow does not work with repositories that contains uppercased chars.

I propose this PR which does the following:

• remove the IMAGE_NAME from the hardcoded env variables
• create an intermadiate step which get the repository and lowercase it, then push it to the GITHUB_ENV
• Finally, use this env variable IMAGE_NAME in the Build and push Docker image step
• (Bonus), use the REGISTRY env as well in the step, (ghcr.io was harcoded)

POC => https://github.com/Nico385412/gitops-cert-level-2-examples/actions/runs/3554828803

[kostis-codefresh]

Should be solved already by #66

Let us know if this doesn't work for you