Wrong sc4h/alpine-s6overlay version in dockerfile

[musicderp]

Building your container as is results in a failure due to the new overlay version (sc4h/alpine-s6overlay:3.12) using ARG OVERLAY_VERSION=3.1.0.1 instead of the one you are using, ARG OVERLAY_VERSION=v2.2.0.3. Changing the first line of /docker/Dockerfile from FROM sc4h/alpine-s6overlay:3.12 to FROM sc4h/alpine-s6overlay:3.11 fixed this for me, but I'm not 100 percent sure how all this stuff works, so I'm reporting it here so you can look at it and possibly fix it. As far as I can tell, this happens because of this latest update to s6overlay https://gitlab.com/sc4h/docker-alpine-s6overlay/-/blob/master/Dockerfile