docs: add hack back in to avoid creating a python module package in API docs, switch to imgmath extension

move conda Dockerfile from gitlab-ci.yml to own file, switch to multi-stage build to slim container size

.gitlab-ci.yml: add docker conda stage, adjust needs for jobs with no dependencies, reduce git depth

cbc_template_fir.condition_psd(): fix off-by-one issue causing 'divide by zero encountered in true_divide' warnings, switch to numpy.inf