Add instructions for migrating from H2 to another database container

docs/operations-guide/running-metabase-on-docker.md

@@ -69,7 +69,33 @@ In this scenario all you need to do is make sure you launch Metabase with the co
 
 Keep in mind that Metabase will be connecting from within your docker container, so make sure that either you're using a fully qualified hostname or that you've set a proper entry in your container's `/etc/hosts file`.
 
-See instructions for [migrating from H2 to MySQL or Postgres](./start.md#migrating-from-using-the-h2-database-to-mysql-or-postgres).
+### Migrating from H2 to Postgres as the Metabase application database
+
+For general information, see instructions for [migrating from H2 to MySQL or Postgres](./start.md#migrating-from-using-the-h2-database-to-mysql-or-postgres).
+
+To migrate an existing Metabase container from an H2 application database to another database container (e.g. Postgres, MySQL), there are a few considerations to keep in mind:
+
+* The target database container must be accessible (i.e. on an available network)
+* The target database container must be supported (e.g. MySQL, Postgres)
+* The existing H2 storage should be mapped outside the running container (refer to the [Docker guide](http://www.metabase.com/docs/latest/operations-guide/running-metabase-on-docker.html#mounting-a-mapped-file-storage-volume) for more information)
+
+The migration process involves 2 main steps:
+
+* Stop the existing Metabase container
+* Run a new, temporary Metabase container to perform the migration
+
+Using a Postgres container as the target, here's an example invocation:
+
+  docker run
+    -v /path/metabase/data:/metabase-data \
+    -e "MB_DB_TYPE=postgres" \
+    -e "MB_DB_DBNAME=metabase" \
+    -e "MB_DB_PORT=5432" \
+    -e "MB_DB_USER=<username>" \
+    -e "MB_DB_PASS=<password>" \
+    -e "MB_DB_HOST=my-database-host" \
+    -e "MB_DB_FILE=/metabase-data/metabase.db" \
+    --name metabase-migration metabase/metabase load-from-h2
 
 ### Setting the Java Timezone
 
@@ -103,12 +129,12 @@ The DB contents will be left in a directory named metabase.db.
 Note that some older versions of metabase stored their db in a different default location.
 
     docker cp CONTAINER_ID:/metabase.db.mv.db metabase.db.mv.db
-    
+
 ### Fixing OutOfMemoryErrors in some hosted environments
 
 On some hosts Metabase can fail to start with an error message like:
 
     java.lang.OutOfMemoryError: Java heap space
-    
+
 If that happens, you'll need to set a JVM option to manually configure the maximum amount of memory the JVM uses for the heap. Refer
 to [these instructions](./start.md#metabase-fails-to-start-due-to-heap-space-outofmemoryerrors) for details on how to do that.