Add Dockerfiles to build minimal Docker

[longsleep]

To reduce the Docker image size a seperate build Dockerfile is
introduced. This Docker image produces a tarball of the released and
compiled software which can when be piped into the Dockerfile.run build
process. The result is a minimal image only containing Spreed WebRTC and
the gear to run OpenSSL.

First create the builder image:

docker build -t spreed-webrtc-builder -f Dockerfile.build .

Next run the builder container, piping its output into the creation of
the runner container:

docker run --rm spreed-webrtc-builder | docker build -t spreed-webrtc -f
Dockerfile.run -

Afterwards run the container like this:

docker run --rm --name my-spreed-webrtc -p 8080:8080 -p 8443:8443 \
    -v `pwd`:/srv/extra -i -t spreed-webrtc

[fancycode]

To make future changes easier to track and get one-line diffs, I would rather write this as

RUN apt-get update && \
    apt-get install -qy \
        autoconf \
        automake \
        build-essential \
        git \
        golang \
        nodejs

[fancycode]

Added various comments.

[icewind1991]

Using apk add --no-cache instead of apk add --update-cache will stop apk from saving the cache which results in a smaller image (since the cache is not stored in the intermediate layer)

[icewind1991]

You could save a bit of work of work keeping the glibc up to date by basing the image on something like frolvlad/alpine-glibc

Or you can probably get rid of glibc altogether by building the server against musl libc in the first place, although the size gain (~7mb iirc) might not be worth the hassle

[longsleep]

Thanks for the feedback, i incorporated all the suggestions and squashed. Looks good now and the updated Dockerfile.run saves almost 20M - awesome!