Building fails with a really bad exception:
```
=> ERROR [linux/386 builder 5/8] RUN go mod download 0.5s
------
> [linux/386 builder 5/8] RUN go mod download:
```
There's some weird bug where building the exporter for `linux/mips64le`
and `linux/arm/v5` has go complaining how
(go-systemd)[https://github.com/coreos/go-systemd/tree/main] has no
linux packages.
We're taking a quick-and-ditry route here and hope that nobody's going
to use the postfix-exporter on MIPS / ARMv5, which hopefully won't be such a big
issue.
We now include logrotate in the the basic image. This does -- up to a
point -- reduce the "separation of concerns", but it simplifies image
management considerably and removes an unneccessary dependency on a
third-party container.
To not repeat outselves, building of postfix-exporter was moved to a
separate action.
Helm now always references the lastest version, as there's no sense in
having specific versions which are all the same.
Lastly, the build fails in the test phase, as it relies on an external
dependency and hence tests have been (temporarily, at least) removed
from the build cycle. This should prove to be a large issue, as we're
building from a fixed SHA hash which we know to be valid.
