add Dockerfiles for container image that includes a (working) fuse-overlayfs

[boegel]

Resulting container images are available through Docker Hub, see https://hub.docker.com/r/eessi/fuse-overlay/tags

To use:

# create temporary directory
export EESSI_TMPDIR=/tmp/$USER
mkdir -p $EESSI_TMPDIR

# create scratch dirs for $HOME, overlayfs and CVMFS
mkdir -p $EESSI_TMPDIR/{home,overlay-upper,overlay-work}
mkdir -p $EESSI_TMPDIR/{var-lib-cvmfs,var-run-cvmfs}

# specify Singularity configuration
export SINGULARITY_HOME="$EESSI_TMPDIR/home:/home/$USER"
export SINGULARITY_BIND="$EESSI_TMPDIR/var-run-cvmfs:/var/run/cvmfs,$EESSI_TMPDIR/var-lib-cvmfs:/var/lib/cvmfs"
export SINGULARITY_CACHEDIR=$EESSI_TMPDIR/singularity_cache

# values to pass to --fusemount in singularity command
export EESSI_CONFIG="container:cvmfs2 cvmfs-config.eessi-hpc.org /cvmfs/cvmfs-config.eessi-hpc.org"
export EESSI_PILOT_READONLY="container:cvmfs2 pilot.eessi-hpc.org /cvmfs_ro/pilot.eessi-hpc.org"
export EESSI_PILOT_WRITABLE_OVERLAY="container:fuse-overlayfs -o lowerdir=/cvmfs_ro/pilot.eessi-hpc.org -o upperdir=$EESSI_TMPDIR/overlay-upper -o workdir=$EESSI_TMPDIR/overlay-work /cvmfs/pilot.eessi-hpc.org"

# start shell in container
singularity shell --fusemount "$EESSI_CONFIG" --fusemount "$EESSI_PILOT_READONLY" --fusemount "$EESSI_PILOT_WRITABLE_OVERLAY" docker://eessi/fuse-overlay:debian10-$(uname -m)

[boegel]

I should probably update the README too, or the docs, with instructions how to use these containers...

[bedroge]

That can probably be part of some software build manual (cleaned up version of your notes/document)? I'll merge this for now, and we can add docs later / in a separate PR.