[GRASS-git] [OSGeo/grass] 952056: docker: update outdated release branch names (#2431)

[Markus Neteler]

  Branch: refs/heads/main
  Home:   https://github.com/OSGeo/grass
  Commit: 9520560b7a9aaab097281cdf7a8bad0559c9d34e
      https://github.com/OSGeo/grass/commit/9520560b7a9aaab097281cdf7a8bad0559c9d34e
  Author: Markus Neteler <neteler at gmail.com>
  Date:   2022-06-17 (Fri, 17 Jun 2022)

  Changed paths:
    M docker/alpine/README.md
    M docker/debian/README.md
    M docker/ubuntu/README.md
    M singularity/debian/README_debian.md

  Log Message:
  -----------
  docker: update outdated release branch names (#2431)

Replaces `releasebranch_7_8` with `releasebranch_8_2`