add docker dev setup

[JannikStreek]

This pull-request features the following:

• Added development setup for docker. A target can be used to select between development and production
• Added a docker compose file for easier setup (also possible to use this for production)
• Integrated pnpm into docker
• For security purposes made the COPY in production more precise; e.g. copy dedicated git files separately instead of copying everything, which might result in security vulnerabilities when accidentally including sensitive files.

closes #6156, #5169

[SamTV12345]

Hi Jannik,

thanks for the pull request. This looks awesome and will definetely help getting started with etherpad. I am currently wondering why the image size increased that much:

Before:

After building with docker build --target production -t etherpad/etherpad:develop .:

[JannikStreek]

@SamTV12345 thanks for the feedback, I will double check where the size is coming from.

[JannikStreek]

@SamTV12345 I think I found the reason, can you check again? RUN chmod -R g=u . increased the image size by 50 mbs...I commented it for now, as it was before. I also added a readme section to explain the compose setup.

[SamTV12345]

Thanks for raising this pull request.