Support lookup of intermediate ID for uidmapping and gidmapping in --userns=auto

[kaivol]

Feature request description

As a rootless user, the host ID in uidmapping and gidmapping in --userns=auto is mapped from the intermediate namespace, just like when using the --uidmap and --gidmap options.

But these options also support preceding the host id in the mapping with the @ symbol, which means that podman will look up the intermediate id corresponding to the given host id and it will map the found intermediate id to the given container id.

I think this functionality would be useful for the uidmapping and gidmapping options in --userns=auto, too.

Suggest potential solution

The behavior should be the same as with the --uidmapping and --gidmapping options (see podman-run -> --uidmap -> Referencing a host ID from the parent namespace).

Have you considered any alternatives?

Looking up the intermediate id manually is also possible, just a little cumbersome.

[giuseppe]

would you like to open a PR to introduce this feature?

[kaivol]

Yes, I would like to look into it.

[rhatdan]

@kaivol you got it.