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/database/testing.go

@@ -0,0 +1,41 @@
+package database
+
+import (
+	"log/slog"
+	"os"
+	"testing"
+
+	"git.eeqj.de/sneak/upaas/internal/config"
+	"git.eeqj.de/sneak/upaas/internal/logger"
+)
+
+// NewTestDatabase creates an in-memory Database for testing.
+// It runs migrations so all tables are available.
+func NewTestDatabase(t *testing.T) *Database {
+	t.Helper()
+
+	tmpDir := t.TempDir()
+
+	cfg := &config.Config{
+		DataDir: tmpDir,
+	}
+
+	log := slog.New(slog.NewTextHandler(os.Stderr, nil))
+	logWrapper := logger.NewForTest(log)
+
+	db, err := New(nil, Params{
+		Logger: logWrapper,
+		Config: cfg,
+	})
+	if err != nil {
+		t.Fatalf("failed to create test database: %v", err)
+	}
+
+	t.Cleanup(func() {
+		if db.database != nil {
+			_ = db.database.Close()
+		}
+	})
+
+	return db
+}