Add a touchfile so the host can determine cluster availability.

Author: FGasper

README.md

@@ -41,6 +41,15 @@ This container stores its state (e.g., mongod data files) under `/data`.
 To introspect that state outside the container, mount a volume to that path
 in the container.
 
+## Readiness checks
+To check whether the cluster is ready, `exec` into the container and check if the
+`ready` file exists. For example:
+```
+while ! docker exec my-container-name [ -e ready ]; do
+    echo "Still waiting for cluster to be ready …"
+done
+```
+
 ## Caveats
 - You **MUST** anticipate the bound ports and export them.
 - The container’s platform will dictate server version availability. For example,

entrypoint.sh

@@ -17,5 +17,7 @@ mlaunch "$@" --bind_ip_all
 echo
 ./print_connstrs.py < data/.mlaunch_startup
 
+touch ready
+
 # Hang forever:
 tail -f /dev/null