Support for Docker image and Yarn version increment

[sonntag-philipp]

Greetings 👋

hope you guys are open for PRs on this project, but I thought this addition would be really useful. In short what I did is:

• Added Dockerfile so we can build an image of the project
• Added nginx config for the image so the image can be hosted in production environments
• Incremented the yarn version so it will install dependencies even faster
• Added packageManager property to the package.json to leverage corepack
• Added automatic script to publish the image using GitHub actions. They are free for FOSS, so I think this is really nice anyway :D For this to work, MercurJS will need an own Docker account.

You can find the published image under "flypsi/mercur-vendor-starter" on Docker Hub. Hope you find this addition helpful :)

[sonntag-philipp]

@agnwoj99 Sorry for the mention. I am just not sure if anyone sees this :D