Add Bullseye build of this image
As a possible solution to lscsoft/lalsuite#643 (closed) I have created a Bullseye branch of this image, see:
https://git.ligo.org/duncanmmacleod/docker-igwn-lalsuite-dev-icc/-/tree/bullseye
and the associated pipeline: https://git.ligo.org/duncanmmacleod/docker-igwn-lalsuite-dev-icc/-/pipelines/488986.
@adam-mercer, please review the Dockerfile
on that branch and give me the OK to push the branch to this project.