Bump deepcell docker image from 0.12.4 -> 0.12.9.

Has to be updated to at least 0.12.7 so that the new fetch_data
utils are available.

Dockerfile

@@ -1,7 +1,7 @@
 # Use vanvalenlab/deepcell-tf as the base image
 # Change the build arg to edit the deepcell version.
 # Only supporting python3.
-ARG DEEPCELL_VERSION=0.12.4-gpu
+ARG DEEPCELL_VERSION=0.12.9-gpu
 
 FROM vanvalenlab/deepcell-tf:${DEEPCELL_VERSION}