add GCP token file to dockerignore

Author: runo721

go-demo/.dockerignore

@@ -6,4 +6,7 @@
 !go.mod
 !go.sum
 # Include our generated version.json file
-!version.json
+!version.json
+
+# Ignore generated credentials from google-github-actions/auth
+gha-creds-*.json