Clean up the unified Dockerfile

[pont-us]

As of version 3.0.0, the Dockerfile is based on a concatenation of several Docker Stacks Dockerfiles as well as AVL-specific content. Some minimal tidying-up has been done (e.g. duplicated LABEL commands and comments removed) but there is still a lot of scope to consolidate steps, which would make the Dockerfile easier to understand, speed up the build, and produce a smaller image. The main candidates for consolidation are the installation steps – currently there are four apt-get installs and three mamba installs!