Rework Dockerfile and prepare for SlackV3 Backend

[duhow]

This is a major rework of the Dockerfile , mainly requires using buildx to mount the wheel packages instead of copying them to the image.

⚠️ ~NOTE: As of https://github.com/errbotio/err-backend-slackv3/commit/908d8229761e09004a843173487d99992f019b62 , slackv3 dependencies cannot be installed - feel free to contribute back with this.~

• Easy way to define extra dependencies for Errbot (irc, xmpp, telegram, others)
• Define BASE_IMAGE
• Use git clone depth to reduce git size
• Stop errbot with SIGINT (actual shutdown of application)
• Compact layers by using less RUN statements

[sijis]

I think once the slackv3 pip package is created, it should just be an update to the pio install command.

[duhow]

By installing backend-slackv3 package from pip I have the following error:

Traceback (most recent call last):
  File "/usr/local/bin/errbot", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/site-packages/errbot/cli.py", line 287, in main
    ep = entry_point_plugins(group="errbot.backend_plugins")
  File "/usr/local/lib/python3.9/site-packages/errbot/utils.py", line 208, in entry_point_plugins
    paths.append(entry_point.dist._path.parent)
AttributeError: 'EntryPoint' object has no attribute 'dist'

[duhow]

Argh. New release. https://pypi.org/project/importlib-metadata/5.0.0/

[sijis]

I have pr #1603 that should address that message. Give it a shot and see if it works.

[duhow]

That seems to work @sijis ! Please review.

[sijis]

Fixed via https://github.com/errbotio/errbot/pull/1608 and #1603.