Add option for "--bind" when running containers

[jbwexler]

Perhaps full customization to allow any singularity flag would be useful.

[yarikoptic]

The likely implementation idea we -- get it from git config but then it is tricky to implement with .datalad/config support since it should come before .git/config... so I think

• just use git config
• if someone wants to hardcode/commit in .datalad/config -- they already can do that by modifying individual invocations.

So we could do it even if no datalad is available etc.

But then we might want to make this added options generic, not necessarily only --bind, so let's make it REPRONIM_CONTAINERS_SING_EXEC_OPTS to add to singularity {cmd}