Update Dockerfile (#2342)

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>

cmd/dendritejs-pinecone/main_noop.go

@@ -1,24 +0,0 @@
-// Copyright 2020 The Matrix.org Foundation C.I.C.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-//     http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-//go:build !wasm
-// +build !wasm
-
-package main
-
-import "fmt"
-
-func main() {
-	fmt.Println("dendritejs: no-op when not compiling for WebAssembly")
-}