docker: Fix fontforge problem

[why]
The docker image can not start fontforge:
  Error relocating /usr/bin/fontforge: FindProgRoot: symbol not found

[how]
The problem might be that we base on the 'latest' Alpine container, but
install a edge fontforge.

If I see this correct:
https://hub.docker.com/_/alpine/tags

'latest' is the same as latest-stable. Probably we should use a matching
apk repo.

Fixes: #1042 (maybe, need to push to test)

Signed-off-by: Fini Jastrow <ulf.fini.jastrow@desy.de>

Dockerfile

@@ -6,7 +6,7 @@ LABEL org.opencontainers.image.title="Nerd Fonts Patcher" \
       org.opencontainers.image.source="https://github.com/ryanoasis/nerd-fonts" \
       org.opencontainers.image.licenses="MIT"
 
-RUN apk update && apk upgrade && apk add --no-cache fontforge --repository=https://dl-cdn.alpinelinux.org/alpine/edge/community && \
+RUN apk update && apk upgrade && apk add --no-cache fontforge --repository=https://dl-cdn.alpinelinux.org/alpine/latest-stable/community && \
     apk add --no-cache py3-pip && \
     pip install configparser