Update Dockerfile and build-docker workflow

PR for #2011.

Changes:

Use Docker Multistage Builds to produce smaller images (398M -> 177M).
Run the container with the bitshares user whose UID is statically set to 10000 and GID to 10001, to follow best practices https://github.com/hexops/dockerfile#use-a-static-uid-and-gid.
Upgrade libraries in the entry script. Although this will bloat the container size (not the image size) and slow down the startup process, it ensures security when https://github.com/bitshares/bitshares-core/issues/2667 is not done.
Push the major.minor version tag to Docker Hub too, to follow best practices https://github.com/hexops/dockerfile#do-not-use-latest-pin-your-image-tags
Update the Docker Hub repository path in the build-docker workflow to a variable instead of hardcoding it.

bitshares / bitshares-core