remove irellevant files and include apt upgrade after apt update in Dockerfile

Files irrelevant for openBIS-20.10.x were removed. The Dockerfile now contains apt-get upgrade after holding postgresql-* packages, since when upgrading also those, we were prompted to decide whether to keep a modified postgres configuration file or not. Preventing the interactive prompt via -o Dpkg::Options::="--force-confold" did not work (we're still prompted). A possible alternative workaround is to set the following environment variable: ENV DEBIAN_FRONTEND=noninteractive before running apt-upgrade.