* Dockerfile.multiarch: use bind- and cache-mounts to speedup build
Use a cache mount for go build cache in the build container and mount
the sources as a bind mount as recommended in
https://docs.docker.com/build/cache/optimize/ this largely speeds up the
container build for subsequent builds.
* Use Dockerfile.multiarch for "dev-docker" target
Let's remove some redundancy. AFAICS the Docker.multiarch does
everything the Docker.linux.* files did. And with the build caches
enable it should be just as quick as building on the host.
* Dockerfile.multiarch: Align the alpine version of the base images
* Dockerfile: Reduce build context by adding more files to .dockerignore
With this commit I have prepared a full basic project structure, so far it doesn't embed phoenix itself and also no kit parts. It includes the whole Drone pipeline to continously publish binaries on push and on tag to our download mirrors (will be available at https://download.owncloud.com/reva/phoenix) and also publishes Docker images at owncloud/reva-phonix. For the Docker image auto_tag is enabled, which means latest is always based on the master branch and tags are automatically converted to minor, major and patch release tags.
