add docker-compose.yml example

[Waldorf3]

Tried this suggested docker-compose.yml but got errors:

jekyll-jekyll-1  | ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux-musl]
jekyll-jekyll-1  | Configuration file: none
jekyll-jekyll-1  |             Source: /srv/jekyll
jekyll-jekyll-1  |        Destination: /srv/jekyll/_site
jekyll-jekyll-1  |  Incremental build: disabled. Enable with --incremental
jekyll-jekyll-1  |       Generating...
jekyll-jekyll-1  |                     done in 0.006 seconds.
jekyll-jekyll-1  |  Auto-regeneration: enabled for '/srv/jekyll'
jekyll-jekyll-1  | jekyll 3.8.6 | Error:  Permission denied @ dir_s_mkdir - /srv/jekyll/_site

Any chance of providing an official docker-compose.yml with the project?