Replace upstream caddy with internal version

[willdollman]

Now that we have a custom-built caddy image, replace the upstream reference with our custom one.

The 269106_2024-04-15_5.3-d8deee19f4ae tag I've used is just from a standard Buildkite run on main - I assume it's just a placeholder that will be overwritten when we run each release?

Confirmed that sg ops update-images works:

$ bazel run //dev/sg -- ops update-images --pin-tag insiders --kind compose --registry internal ~/code/deploy-sourcegraph-docke
r/docker-compose/

[...]
Updated caddy
  - docker.io/sourcegraph/caddy:269106_2024-04-15_5.3-d8deee19f4ae@sha256:13b9ee03ca9feba90d83129922f96ee9c1b40064a048b73feac3a334e27c3aa1
  + us.gcr.io/sourcegraph-dev/caddy:insiders@sha256:13b9ee03ca9feba90d83129922f96ee9c1b40064a048b73feac3a334e27c3aa1
[...]

Checklist

• Sister deploy-sourcegraph change:
• Sister customer-replica change (if necessary, for any changes affecting pure-docker or configuration):
• All images have a valid tag and SHA256 sum

Test plan

• Tested running the docker-compose setup locally with new caddy image
• Tested running the pure-docker setup locally with new caddy image
• Tested sg ops update-images