Map volumes to persistent paths in example compose files

[he3als]

The documentation currently includes example docker-compose.yml files that will clear users' NPM data when the server is re-deployed, which can confuse, e.g., this Reddit post.

This pull request makes the example compose files instead save to a persistent location, /srv/nginx-proxy-manager.

[nginxproxymanagerci]

Docker Image for build 1 is available on DockerHub as jc21/nginx-proxy-manager:github-pr-3654

Note: ensure you backup your NPM instance before testing this PR image! Especially if this PR contains database changes.

[jc21]

I disagree with this change. The docker-compose example will create persistent, mounted volumes relative to the CWD when the compose up is run. The only way these volumes are wiped out is when running compose down or if the CWD is itself a temporary FS.

The reddit post does not determine why mounting an absolute folder worked for them.