Add a way to use Docker for building everything

I’d like to test #751 on my own instance, but installing all the build
dependencies on my server isn’t ideal - having this script in the repo
would make it possible to simply point my compose file to the git repo
and have it build the Linux binary itself.

Note that it uses a somewhat “inefficient” builder step, i.e. not
combining steps together to reduce layers, as it uses a multi-stage
build to have a lean final image. This makes it easier to re-build if
something needs to change, as the cache is used more optimally.

For example, if only some go files change, most of the build is already
cached and only the go step gets re-run.

The more “efficient” builder step would look like this, but would have
to build the docs, web app and go CLI for any change in any file:

```Dockerfile
FROM golang:1.19-bullseye as builder

RUN apt-get update && \
    curl -fsSL https://deb.nodesource.com/setup_18.x | bash && \
    apt-get install -y \
    build-essential \
    nodejs \
    python3-pip

WORKDIR /app
ADD . .

RUN make web docs cli-linux-server
```

docs/develop.md

@@ -163,6 +163,15 @@ $ make release-snapshot
 
 During development, you may want to be more picky and build only certain things. Here are a few examples.
 
+### Build a Docker image only for Linux
+
+This is useful to test the final build with web app, docs, and server without any dependencies locally
+
+``` shell
+$ make docker-dev
+$ docker run --rm -p 80:80 binwiederhier/ntfy:dev serve
+```
+
 ### Build the ntfy binary
 To build only the `ntfy` binary **without the web app or documentation**, use the `make cli-...` targets: