Switching from Alpine Image to multistaged opensuse/leap - #2347

BusyBox `adduser` was reading config from `/etc` that unnecessarily populates the home folder with various dot files.

Alternative approach is to create the user and home folder manually. This avoids some extra files like the `-` suffixed backup copies from `adduser`.

The additions from `adduser` reading `/etc` does not appear to apply if the directory already exists, and permissions (including SGID) are adjusted properly for the home dir.

This mean the excess backup copies in `/etc` are introduced again however.

- Removing bulk of the noise from inline documentation.
- Packages bundled into single line like previous the Alpine version had.
- `CACHE_ZYPPER` is only used as an `ARG` in the `builder` stage.
- `zypper clean` wasn't able to clear anything from the install root, other than the `CACHE_ZYPPER` mount.

These have been available via opt-in prior to v23 of Docker Engine with `DOCKER_BUILDKIT=1` ENV as a prefix to running `docker build`, however it's been requested to avoid the syntax.

No HereDoc (multi-line RUN with EOF marker) or `RUN --mount` available. This makes the `busybox` approach a hassle, so I've brought back the explicit creation of user and home dir.

Without the cache mounts, bring back `zypper clean`. It's not doing much as the `--cache-dir` is still set, but should reduce disk space for the `builder` layer. Local builds will be slower as a result when this layer is invalidated.

AFAIK, this also makes it tricky to use the `ZYPPER_OPTIONS`? So no longer DRY.

…suse

refactor(dockerfile): Change base Alpine (3.17) => openSUSE Leap (15.4)