rebase: apply audit bug fixes on latest main

Rebased fix/1.0-audit-bugs onto current main (post-merge of PRs #119, #127).

Changes:
- Custom domain types: docker.ImageID, docker.ContainerID, webhook.UnparsedURL
- Type-safe function signatures throughout docker/deploy/webhook packages
- Remove docker-compose.yml, test helper files
- README and Dockerfile updates
- Prettier-formatted JS files

internal/docker/types.go

@@ -0,0 +1,7 @@
+package docker
+
+// ImageID is a Docker image identifier (ID or tag).
+type ImageID string
+
+// ContainerID is a Docker container identifier.
+type ContainerID string