Updates/adds a new multistage (build-kit) Dockerfile. (if accepted,
could make `Dockerfile.monolith` and `Dockerfile.polylith` in
`build/docker` obsolete)
There's no huge difference between the dockerfiles, except this uses a
non-root user when running the container, also doesn't copy the working
directory to the image when building.
Also adds vulnerabilities scans using
[Trivy](https://github.com/aquasecurity/trivy) for the created docker
images. (untested)
Building images is done using
```
docker build . --target image-monolith -t dendrite-monolith
docker build . --target image-polylith -t dendrite-polylith
```
As noted in the comments, only adds `dendrite-polylith-multi` to the
polylith image and all required binaries to the monolith image.
Probably needs some docs updating, if this is accepted.
Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>
