Add comment regarding docker owning migrations files

documentation/docker.md

@@ -48,3 +48,9 @@ If you have issues at some point with `__pycache__` files, you can delete them:
 ```bash
 find . | grep -E "(__pycache__|\.pyc|\.pyo$)" | sudo xargs rm -rf
 ```
+
+## Issues with Django migrations
+
+With the docker setup, Django migrations are created from within the container and as a result those files are own by the docker user.
+
+If you have issues with git regarding the migrations files (i.e. they don't disappear when you change branch), you need to become an owner of the files: `chown -R $(id -u):$(id -g) backend`.