[WIP] gdal2.4.2-py3.6

[dazza-codes]

• don't merge this PR, just add a new branch for it
  • cherry-pick stuff to other branches
  • leaving the WIP status indefinitely to block an accidental merge
• use gdal and python versions in the docker image tag
• install cython, numpy, gdal, rasterio into the PYTHONPATH
• make rules to build and package a lambda layer using /opt
  • requires an ARG to define the install PREFIX as /opt
  • use of ARG could be generalized for parameterized builds (not done here)
• cannot update the master README on this PR

TODO

• check the package works as a lambda layer
  • check the extraction to /opt is "OK"
  • check the ldd on some .so libs works with LD_LIBRARY_PATH
  • seems to work OK, although it might clobber stuff (?) don't care to fix it at present
• prune the package of extraneous files
  • optional optimization to keep the package size down

related issues/PRs

• Set PREFIX through ARG #18 Allow setting PREFIX at build time #19 - use an ARG to set the PREFIX (default to /var/task)
• Create a lambda layer + doc #11 Correct paths resulting package.zip #17 - lambda layer package.zip

[vincentsarago]

FYI, I'm on holiday right now and trying to stay as far as possible from my laptop. I'll have a look later.