Merge pull request #5476 from joshuashort/docker-h2-migration-instructions

Docker H2 to Postgres migration documentation

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

@@ -71,7 +71,35 @@ 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 database should be [mapped outside the running container](#mounting-a-mapped-file-storage-volume)
+
+The migration process involves 2 main steps:
+
+1. Stop the existing Metabase container
+2. Run a new, temporary Metabase container to perform the migration
+
+Using a Postgres container as the target, here's an example invocation:
+
+    docker run --name metabase-migration \
+        -v /path/metabase/data:/metabase-data \
+        -e "MB_DB_FILE=/metabase-data/metabase.db" \
+        -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" \
+        metabase/metabase load-from-h2
+
+To further explain the example: in addition to specifying the target database connection details, set the `MB_DB_FILE` environment variable for the source H2 database location, and pass the argument `load-from-h2` to begin migrating.
 
 ### Setting the Java Timezone