rework: address review feedback on PR #126

Changes per sneak's review:
- Delete docker-compose.yml, add example stanza to README
- Define custom domain types: ImageID, ContainerID, UnparsedURL
- Use custom types in all function signatures throughout codebase
- Restore imageID parameter (as domain.ImageID) in deploy pipeline
- buildContainerOptions now takes ImageID directly instead of
  constructing image tag from deploymentID
- Fix pre-existing JS formatting (prettier)

make check passes with zero failures.

internal/domain/types.go

@@ -0,0 +1,16 @@
+// Package domain defines domain-specific string types for compile-time safety.
+// Using named types prevents accidentally passing the wrong string argument
+// (e.g. a container ID where an image ID is expected).
+package domain
+
+// ImageID is a Docker image identifier (ID or tag).
+type ImageID string
+
+// ContainerID is a Docker container identifier.
+type ContainerID string
+
+// UnparsedURL is a URL stored as a plain string without parsing.
+// Use this instead of string when the value is known to be a URL
+// but should not be parsed into a net/url.URL (e.g. webhook URLs,
+// compare URLs from external payloads).
+type UnparsedURL string