Dependencies and docker multi stage

[jonasbn]

Issue #53 describes an issue with ignoring Markdown regions with code fences. This was an issue in pyspelling, which is the core component in this action. Luckily @facelessuser, the maintainer of pyspelling was to fix it. So the requirement for pyspelling was bumped from 2.6.1 to 2.7.3

This however demonstrated that several of the dependencies was not up to date.

'The following dependencies have been updated:

• backrefs updated from 4.5 to 5.0.1
• bracex updated from 2.0.1 to 2.1.1
• lxml updated from 4.6.0 to 4.6.3
• Markdown updated from 3.3.1 to 3.3.4
• pymdown-extensions updated from 8.1 to 8.2
• pyspelling updated from 2.6.1 to 2.7.3
• PyYAML updated from 5.3.1 to 5.4.1
• six updated from 1.15.0 to 1.16.0
• soupsieve updated from 2.0.1 to 2.2.1
• wcmatch updated from 7.1 to 8.2

Additionally some work has been done in regard to the Docker build, so it is possible to specify new languages, using multi-stage build, see PR #39 addressing issue: #13

Please see the Wiki for details.