Add nss_wrapper to alpine images to run container with different user

[wolfgangwalther]

nss_wrapper is now available in alpine's edge/testing repo: https://pkgs.alpinelinux.org/packages?name=nss_wrapper&branch=edge&repo=&arch=

Adding this to the alpine variants will allow to run those with different user ids, too - similar to the debian images.

Ref #253 and #359.

The docs will need adjustment, too.

[yosifkit]

Mixing Alpine's "edge" with a released version does not go well in our experience 🙈. It is not a supported configuration from Alpine's perspective: https://gitlab.alpinelinux.org/alpine/apk-tools/-/issues/10699.

The next release of Alpine should be in November/December, so we can revisit this then if it moves to the stable release.