Fix binding of folders when running with singularity.

[njohner]

When running with singularity the current working directory and home folders get mounted by default. It seems that this can collide with our own mounts, which get mounted into webapp/assets of the zDB directory, which usually also is the CWD. I did not quite understand why this works in certain cases but not in others, but it seems this led to zDB not running with singularity on our server. Adding the -c option to avoid sharing the filesystem resolves this issue.

Checklist

• Changelog entry
• Check that tests still pass
• Add tests for new features and regression tests for bugfixes whenever possible.