Refactor Dockerfile

[PeterDaveHello]

1. Use apk -U add instead of apk update + apk add
2. Separate package installation and user, group setup, the combination
   to reduce a docker image layer didn't bring benefits here.

[cez81]

Maybe sync this with #1615

[sapk]

I was planning and updating and cleaning Dockerfile for alpine part, following https://github.com/gliderlabs/docker-alpine/blob/master/docs/usage.md, for example adding also --no-cache instead of rm -rf /var/cache/apk/*. Has you like this could be merged before #1615 because in this PR I will add multi-arch source (FROM) and multi-stage to have one Dockerfile for every platform and this need time to do it ^^.

[PeterDaveHello]

Okay, just to confirm, so what I need to do here is to use --no-cache instead of manually cache cleaning and @sapk you'll do the multiarch part? Thanks.

[sapk]

Please add --no-cache and I will L-G-T-M since my change need a function not yet implemented in stable release of docker.

I would also prefer to not add a stage by splitting RUN instruction and keep it together.

[PeterDaveHello]

What's the reason why here to combine two unrelated things into a single layer? I didn't see a good point here.

[bkcsoft]

I'm fine with this change. Concatenating the layers only makes sense if we're removing build-deps in the end. LGTM

[sapk]

LGTM
For multiple RUN/layer, I tend to keep things together but I am not enforcing it ^^

[sapk]

Oh I forget the -U is useless with --no-cache but this could also be merged with it.