Dockerfile rewrite for Alpine

[neteler]

edited description by @mmacata below to not overwrite @netelers description

• use of GRASS GIS / PDAL alpine docker base image
• mainly done up to line 106

TODO:

• SNAP part should come from here: https://github.com/mundialis/esa-snap (currently commented out)
• update requirements.txt to match Python 3.7 as needed

[mmacata]

Update to new alpine packages (gdal, ...) and more requirements
Removed snappy as this should be added from mundialis/esa-snap

[mmacata]

This PR adds the long-waited-for alpine Dockerfile for actinia-core. It uses multistage build:

• FROM mundialis/grass-py3-pdal:stable-alpine as grass
• FROM mundialis/esa-snap:latest as snap
• Dockerfile-build-pkgs as new image containing runtime requirements (apk + pip)
• Dockerfile-runtime-pkgs as new image containing runtime requirments (apk + pip)

Additionally there is Dockerfile_alpine_full, combining build-and runtime pkgs and the short Dockerfile. To be discussed if we keep this because it builds much slower as requirements all need to be fetched.

[neteler]

Great work, @mmacata, thanks a lot!

[neteler]

FYI: Fixes #53