Update Web Portal dependencies for improve-build branch

[grantfitzsimmons]

Most of this work was already done by @sglim2 at Cardiff University:

Briefly, the changes include:

• python2->python3: Python code has been updated to python3, (3.8 and 3.9 tested). This is mostly changing print statements, and explicitly stating the text encoding in elementtree write statements.
• solr7.5.0->solr8.11.1: Updated the hard-coded solr version to 8.11.x (current download is 8.11.1)
• Dockerfile: I've added a Dockerfile to this branch, based on https://github.com/specify/specify7-docker/blob/master/Dockerfile.
  (Note, I've hard-coded my forked/branched github repo, which would need to be changed.)
• README: Updated the readme file to reference the new python and solr versions.