Update dockerfile build: new python base image, reduce disk space use Use newest python:3-alpine. Remove sphinxcontrib libraries and xapian docs that were left around from the build saving 2M.

try:
    # Python 3+
    from time import perf_counter
except (ImportError, AttributeError):
    # Python 2.5-2.7
    from time import clock as perf_counter   # noqa: F401