Add GitHub Actions workflow for GKE deployment

[ghost]

This workflow builds a Docker container, publishes it to Google Container Registry, and deploys it to GKE on pushes to the 'main' branch. It includes steps for authentication, Docker image building, and deployment configuration.

Trac ticket number

ticket-XXXXX

Branch description

Provide a concise overview of the issue or rationale behind the proposed changes.

Checklist

• This PR targets the main branch.
• The commit message is written in past tense, mentions the ticket number, and ends with a period.
• I have checked the "Has patch" ticket flag in the Trac system.
• I have added or updated relevant tests.
• I have added or updated relevant docs, including release notes if applicable.
• I have attached screenshots in both light and dark modes for any UI changes.