build.sh: print the squashed image ID to stdout

Now the output would look like:
...
Successfully built 145d36ab3a06
Squashing the image '145d36ab3a06' from 'centos/s2i-base-centos7' layer.
Squashed as '4f52ad3cc7ef1f19951f9b6a147823fe5ea4a4baea3a70627385fc04bb78f73d'.
...

Fixes: sclorg#103

build.sh

@@ -152,9 +152,11 @@ squash ()
 
   base=$(awk '/^FROM/{print $2}' "$1")
 
-  echo "Squashing the image '$unsquashed' from '$base' layer"
+  echo "Squashing the image '$unsquashed' from '$base' layer."
   IMAGE_ID=$("${PYTHON-python3}" "$script_dir"/squash.py "$unsquashed" "$base")
 
+  echo "Squashed as '$IMAGE_ID'."
+
   echo "$unsquashed" > .image-id.squashed_from
   echo "$IMAGE_ID" > .image-id.squashed
 }