Add Operator image build and publish to the pipeline

[ciarams87]

The Operator image needs to published to a public repository for certification.

We need to add a step in our pipeline to build the Operator image and publish it to GHCR along with our other images.

Acceptance criteria:

• The Operator image is built in the pipeline
• Its repo is docker pull ghcr.io/nginx/nginx-gateway-fabric/operator
• It should be pushed on pushed to main with an edge tag similar to the other images, and with the appropriate tag on releases