Add Gitlab Container Registry

[eminks]

I have tested cosign for gitlab container registry and work!

My verify result:

$ cosign verify -key cosign.pub registry.gitlab.com/eminks/denemler:latest
The following checks were performed on each of these signatures:
  - The cosign claims were validated
  - The signatures were verified against the specified public key
  - Any certificates were verified against the Fulcio roots.
  - WARNING - THE CERTIFICATE EXPIRY WAS NOT CHECKED. set COSIGN_EXPERIMENTAL=1 to check!
{"Critical":{"Identity":{"docker-reference":""},"Image":{"Docker-manifest-digest":"sha256:c93fa5b332de47c9f031284c7aa7a95988536e515571b73fa6294a0f54b289aa"},"Type":"cosign container signature"},"Optional":null}

Based on: #40

[dlorenc]

Thanks! This is great!