DX-118079: Remove Docker build from GitHub Actions (#98)

Removes the docker.yaml workflow entirely. Docker image build, tag,
and push is now handled by the Jenkins mcp-image-build job, which
polls dremio-mcp every 5 minutes via SCM polling using the existing
github-app-key credential. No credentials or registry paths remain
in the OSS repo.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ssaumitra