Fix local experiment docs to use gcr.io/fuzzbench registry. (google#782)

* Fix local experiment docs to use gcr.io/fuzzbench registry.

Right now, custom registry e.g. local-server:5000 don't work
due to missing dispatcher-image instructions and missing support
in make. For now, just recommend using gcr.io/fuzzbench, this
still keeps images locally.

* Add FIXME.

docs/running-a-local-experiment/running_a_local_experiment.md

@@ -45,8 +45,10 @@ trials: 5
 # 1 day = 24 * 60 * 60 = 86400
 max_total_time: 86400
 
-# The location of your docker registry.
-docker_registry: lab-server:5000
+# The location of the docker registry.
+# FIXME: Support custom docker registry.
+# See https://github.com/google/fuzzbench/issues/777
+docker_registry: gcr.io/fuzzbench
 
 # The local experiment folder that will store most of the experiment data.
 # Please use an absolute path.