Update the instruction for using the docker based development environment (#3395)
diff --git a/docker/scripts/dev-env.sh b/docker/scripts/dev-env.sh
index 17e37ed..63eec60 100755
--- a/docker/scripts/dev-env.sh
+++ b/docker/scripts/dev-env.sh
@@ -30,10 +30,15 @@
# bazel build --config=ubuntu heron/...
#
# To enter an existing container with a new shell, find the container
-# id with this command:
+# ID with this command first:
# docker ps -a
-# And then :
+# The image name looks like: "heron-dev:ubuntu18.04".=
+# After the container is found, execute the following commands to start
+# the container in case it is not started yet, and then start a new
+# terminal in the container:
+# docker container start CONTAINER_ID
# docker exec -it CONTAINER_ID bash
+#
set -o nounset
set -o errexit
