lemmy/docs/src/contributing_docker_development.md

Run:

git clone https://github.com/dessalines/lemmy
cd lemmy/docker/dev
./docker_update.sh # This builds and runs it, updating for your changes

and go to http://localhost:8536.

Note that compile times are relatively long with Docker, because builds can't be properly cached. If this is a problem for you, you should use Local Development.