chore: Add GitHub Container Registry as alternative image source

[twk3]

Add documentation for using GitHub Container Registry (GHCR) as an alternative to AWS ECR for accessing Currents container images.

Checklist

• PR title is Changelog-friendly: (fix|feat|chore): title [CSR-xxx]
• User-friendly and meaningful description of the changes
• Documentation was updated

Summary

• Adds new "Alternative: GitHub Container Registry" section to docs/container-images.md
• Documents the prerequisite of being invited to the private currents-dev/customer-images repository
• Provides step-by-step instructions for GitHub PAT creation and GHCR authentication
• Includes Docker Compose configuration example with DC_CURRENTS_IMAGE_REPOSITORY=ghcr.io/currents-dev/currents-
• Adds mirroring instructions specific to the GHCR image naming convention

Review Hints

• Single file change: docs/container-images.md
• The new section is appended after the existing AWS ECR documentation
• Note the different image naming pattern: GHCR uses currents- prefix (e.g., currents-api) vs ECR's path-based naming

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Summary by cubic

Adds GHCR as an alternative source for Currents container images, with auth steps, Docker Compose config, and mirroring guidance. This helps customers who can’t use AWS IAM to pull images reliably.

• Migration
  • Get invited to github.com/currents-dev/customer-images.
  • Create a GitHub PAT with read:packages and run docker login to ghcr.io.
  • Set DC_CURRENTS_IMAGE_REPOSITORY=ghcr.io/currents-dev/currents- and choose a tag (e.g., staging).
  • Mirror images to your own registry if needed; note GHCR uses the currents- prefix (e.g., currents-api).

^{Written for commit 06e8842. Summary will update on new commits.}

[cubic-dev-ai]

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