moved adding label about base image digest to CI steps

Dockerfile

@@ -1,9 +1,7 @@
 FROM ghcr.io/mlflow/mlflow:latest
-ARG BASE_IMAGE_DIGEST=unknown
 
-# Set a label with the base image digest for traceability
-LABEL base-image.digest="${BASE_IMAGE_DIGEST}" \
-    build-date="$(date -u +'%Y-%m-%dT%H:%M:%SZ')" \
+# Set a labels for traceability
+LABEL build-date="$(date -u +'%Y-%m-%dT%H:%M:%SZ')" \
     maintainer="Brian Jensen"
 
 # Use --no-cache-dir to avoid storing pip cache in the image