Numba caching fails on Docker

[AlecThomson]

From @axshen:

I get this error RuntimeError: cannot cache function 'gaussft': no locator available for file '/opt/conda/envs/racs-tools/lib/python3.8/site-packages/racs_tools/gaussft.py' when running beamcon_2D

Looks like this is the same issue as numba/numba#4032.

Fix is in the above issue, which is to add:

RUN mkdir /tmp/numba_cache & chmod 777 /tmp/numba_cache & NUMBA_CACHE_DIR=/tmp/numba_cache
ENV NUMBA_CACHE_DIR=/tmp/numba_cache

to the Dockerfile.