Running `nsenter` with `-U` fails:
```
nsenter: setns(): can't reassociate to namespace 'user': Invalid argument
```
This is probably because the container runs in the same user namespace
as the host anyway, so it really doesn't matter.
The `-a`/`--all` argument for `nsenter` is present in the *util-linux*
version of `nsenter`, but not the BusyBox version. The latter is
present in Alpine, so we need to adjust our wrapper script to use
compatible syntax, which means explicitly listing the namespaces we want
to enter.
The configuration system uses `curl` to fetch _age_ encryption keys from
the key server. It was installed by default in the Fedora Minimal
container image, but it is not included in the base Alpine image.
Using Alpine instead of Fedora Minimal reduces the size of the final
image from 175 MB to 50 MB. This should speed up execeution on slow
machines like Raspberry Pis.
