config: please implement support for "hostname" parameter

[onlyjob]

As a equivalent to --hostname singularity's command line option.

Currently hostname is not supported in config section:

#011* Invalid label: No argument or block type is named "hostname"

Thanks.

[onlyjob]

Currently can be set using environment variable inside task block:

env {
    SINGULARITY_HOSTNAME = "singhost"
}

See Singularity’s environment variables reference:
https://sylabs.io/guides/3.4/user-guide/appendix.html