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/service/deploy/deploy_container_test.go

@@ -0,0 +1,44 @@
+package deploy_test
+
+import (
+	"context"
+	"log/slog"
+	"os"
+	"testing"
+
+	"git.eeqj.de/sneak/upaas/internal/database"
+	"git.eeqj.de/sneak/upaas/internal/models"
+	"git.eeqj.de/sneak/upaas/internal/service/deploy"
+)
+
+func TestBuildContainerOptionsUsesImageID(t *testing.T) {
+	t.Parallel()
+
+	db := database.NewTestDatabase(t)
+
+	app := models.NewApp(db)
+	app.Name = "myapp"
+
+	err := app.Save(context.Background())
+	if err != nil {
+		t.Fatalf("failed to save app: %v", err)
+	}
+
+	log := slog.New(slog.NewTextHandler(os.Stderr, nil))
+	svc := deploy.NewTestService(log)
+
+	const expectedImageID = "sha256:abc123def456"
+
+	opts, err := svc.BuildContainerOptionsExported(context.Background(), app, expectedImageID)
+	if err != nil {
+		t.Fatalf("buildContainerOptions returned error: %v", err)
+	}
+
+	if opts.Image != expectedImageID {
+		t.Errorf("expected Image=%q, got %q", expectedImageID, opts.Image)
+	}
+
+	if opts.Name != "upaas-myapp" {
+		t.Errorf("expected Name=%q, got %q", "upaas-myapp", opts.Name)
+	}
+}