portable_mtime for py3_image

# 🚀 feature request

### Description

When using `py3_image` the _mtime_ (modification time) for all the files are set to 0 unix time (cira 1970). Some python libraries panic when they see that (eg. `zipfile` python builtin lib `ValueError: ZIP does not support timestamps before 1980`)

### Describe the solution you'd like

`container_image` has an option to set all mtimes to a fixed valued: see [`portable_mtime`](https://github.com/bazelbuild/rules_docker/blob/8e70c6bcb584a15a8fd061ea489b933c0ff344ca/container/layer.bzl#L355) but IIUC it needs to be routed to `py3_image` through the signatures somehow, hence this feature request.

Basically can we by default or through an option in `py3_image` set all the mtimes to something other than `0` ?

Start with the portable_mtime option in container/layer.bzl and trace how py3_image constructs its image and forwards rule options. Determine where the option would enter the py3_image signature and how its default should behave. Done means py3_image can produce files with a nonzero fixed mtime so Python's zipfile accepts them.

Written by the indexing model from the issue text.