I’ve used lemmy_migrate in the past and it’s good for one-way copy. I’ve also seen lasim, but I haven’t tried it. You may find other options on awesome-lemmy.

That’s it! If you don’t specify a host path, i.e. the path before the colon, Docker will create an volume which saves any changes you make to that path in the container, but won’t mount any existing path from the host to the container.