fix: use imageID in createAndStartContainer (closes #124)

Wire the imageID parameter (returned from docker build) through
createAndStartContainer and buildContainerOptions instead of
reconstructing a mutable tag via fmt.Sprintf.

This ensures containers reference the immutable image digest,
avoiding tag-reuse races when deploys overlap.

Changes:
- Rename _ string to imageID string in createAndStartContainer
- Change buildContainerOptions to accept imageID string instead of deploymentID int64
- Use imageID directly as the Image field in container options
- Update rollback path to pass previousImageID directly
- Add test verifying imageID flows through to container options
- Add database.NewTestDatabase and logger.NewForTest test helpers

internal/logger/testing.go

@@ -0,0 +1,11 @@
+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),
+	}
+}