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/logger/testing.go

@@ -1,11 +0,0 @@
-package logger
-
-import "log/slog"
-
-// NewForTest creates a Logger wrapping the given slog.Logger, for use in tests.
-func NewForTest(log *slog.Logger) *Logger {
-	return &Logger{
-		log:   log,
-		level: new(slog.LevelVar),
-	}
-}