Unable to run host commands with singularity in a `guix pack -f squashfs` conainer

[Sebastian Gibb via <help-guix@gnu.org>]

Hi,

I am trying to use `guix pack` to create a reproducible container that could be
used on an HPC. Unfortunately I am not able to schedule jobs from within the
container or run any command from the host system.

The HPC uses Cent OS 8.1. It offers slurm 20.02.5 for job scheduling and
singularity 3.4.2 for virtualisation/own software bundles.

I generated my container as follows:
```
cp $(/usr/local/bin/guix time-machine --commit=c78d6c6 -- pack \
	--relocatable --relocatable \
	--format=squashfs \
	--entry-point=bin/bash \
	--symlink=/bin=bin \
	--symlink=/lib=lib \
	--symlink=/share=share \
	--save-provenance \
    bash coreutils) mwe.squashfs
```

Next I try to use the host's `ssh` or slurms `sinfo`/`sbatch` from within the
container but I got the error `No such file or directory`. Using `ls` or `cat` I
could access these files but I am not able to execute them:

```
SINGULARITY_BIND="/usr" singularity run mwe.squashfs
> /usr/bin/ssh
runscript: /usr/bin/ssh: No such file or directory

> ls /usr/bin/ssh
/usr/bin/ssh

> cat /usr/bin/ssh
ELF>�@($
        @8
...
```

I could workaround the `ssh` problem by putting `openssh-sans-x` into the
`guix pack` command and binding `/etc/group`, `/etc/passwd` and `/var/run` to
my singularity container. But if I include `slurm` I always got an error for
`sinfo`/`sbatch`:
"slurm_partitions: Zero Bytes were transmitted or received"
(maybe I need to bind some more pathes to singularity?)

1. Is there a way to use the host commands from within the singularity/squashfs
container generated by `guix pack`?
2. Or can I bind some more files/directory to my singularity command to get
`slurm` commands working?

By the way: I am not able to modify the PATH variable with `guix pack` generated
containers. It seems that SINGULARITY_PREPEND_PATH/SINGULARITY_APPEND_PATH are
ignored:
```
> SINGULARITY_PREPEND_PATH="/usr/bin" \
    singularity exec container/mwe.squashfs /bin/bash -c 'echo $PATH'
WARNING: passwd file doesn't exist in container, not updating
WARNING: group file doesn't exist in container, not updating
/gnu/store/266jw5fcbygya3fkfbxkaa4yl23hrwci-profile/bin
```

3. How to modify the PATH variable?
(I know I could create a wrapper start script like
`export PATH=$PATH:/usr/bin; /bin/bash` but even than I can't execute
`ssh`/`sinfo`/`sbatch` and other host commands.)

Best wishes,

Sebastian