[docker] modify the Dockerfile so that the user is logged in as a non-root user

named "dnadna" by default

it also makes some additional concessions to allowing the user to mount a local directory as the dnadna source directory (which is now in /home/dnadna/dnadna instead of /src/dnadna). If the special file /home/dnadna/dnadna/.docker does not exist, then it means the user may have overridden this directory with a mountpoint, in which case the package may have to be reinstalled with pip