Add configuration option to force docker pull

docs/trains.conf

@@ -83,6 +83,10 @@ agent {
     # apt cache folder used mapped into docker, for ubuntu package caching
     docker_apt_cache = ~/.trains/apt-cache
 
+    # set to true in order to force "docker pull" before running an experiment using a docker image.
+    # This makes sure the docker image is updated.
+    docker_force_pull: false
+
     default_docker: {
         # default docker image to use when running in docker mode
         image: "nvidia/cuda"