Add external-dns image.

[dlorenc]

Fixes:

Related:

Pre-review Checklist

• IMPORTANT: 'image-request' tag has been applied if this PR is adding any images, including new versions or variants

For new image PRs only

If you have an apko.yaml file in this PR you need to follow this checklist, otherwise feel free to remove.

• Image is marked experimental or stable as appropriate
• The last two minor versions are available
• The latest tag points to the newest stable version
• There is a dev tag available that includes a shell and apk tools (by depending on 'wolfi-base')
• The image runs as nonroot and GID/UID are set to 65532
  • Alternatively the username and GID/UID may be a commonly used one from the ecosystem e.g: postgres
  • See above for exceptions to nonroot rule
• X ] ENTRYPOINT
  • For applications/servers/utilities call main program with no arguments e.g. [redis-server]
• CMD:
  • For utilities/tooling bring up help e.g. –help
• Consider where and how the image deviates from popular alternatives. Is there a good reason and is it documented?
• Add annotations e.g:

annotations:
  "org.opencontainers.image.authors": "Chainguard Team https://www.chainguard.dev/"
  "org.opencontainers.image.url": https://edu.chainguard.dev/chainguard/chainguard-images/reference/busybox/ # use the academy site here
  "org.opencontainers.image.source": https://github.com/chainguard-images/images/tree/main/images/bazel # use github here

• Check if environment variables are needed e.g. to set data locations
• Ensure the image responds to SIGTERM
  • docker kill $(docker run -d --rm cgr.dev/chainguard/nginx)
• Documentation. Let's make this excellent. Include usage example.
• Error logs write to stderr and normal logs to stdout. DO NOT write to file.
• Include tests, at the very least a basic smoke test.