Explicitly set the alpine version in Dockerfile.

[thibwk]

Otherwise docker build fails with:

[...]
fetch https://dl-cdn.alpinelinux.org/alpine/edge/community/x86_64/APKINDEX.tar.gz
ERROR: unable to select packages:
  texmf-dist-science (no such package):
    required by: world[texmf-dist-science]
The command '/bin/sh -c apk --no-cache add     inkscape     make     ncurses     pandoc-cli     perl     py3-pygments     py3-boto3     py3-requests     python3     texlive     texlive-luatex     texmf-dist     texmf-dist-formatsextra     texmf-dist-latexextra     texmf-dist-pictures     texmf-dist-science     wget' returned a non-zero code: 1

[thibwk]

This was just done by #542