Add docker-py for Alpine

Add option to set the strftime format

Add commandline option for strftime_format. Set check_updates in config because otherwise outdated fails since it isn't set even though there is a 'global'

Readd armv6 docker image

- See #1762, thanks to @eifinger

ci: fix release process and improve build speeds

Documentation to password protect Dockerized Glances

Docker Compose's version of Secrets does not yet support ``uid`, ``guid``, or ``mode``.

fix: docker dev build contains all optional requirements

- requires base image to be full fledged python -> results in bigger image size for dev.

docs: fix simple typo, optionnal -> optional

[WebUI] Fix display of null values for GPU plugin

There is a small typo in glances/static_list.py.

Should read `optional` rather than `optionnal`.