Document how to customise an image (optionally with templating)

Since this has only been discussed in a few issues (I rememeber some older discussions on gitlab) or discord, it could be useful to add a section in the documentation on how to precisely customize images for different file support.

We had an idea of adding some overhead during the pipeline runs to _inject_ (install) blender on each build,_this is the plugin system from Webber's comment above_. Until we have support for this (if we accept to go that way), I think including an easy to follow guide in the documentation could be helpful for many.

Right now, all we have is this:
https://game.ci/docs/docker/customize-docker-images

It is helpful, but I think it can be improved. Just another idea on top of this, we could use the power of `mdx` (react in markdown files) to allow people to pick their unity version, the tool they want and generate some ready to use scripts to publish what they need.

Example: A react component that generates a script from a form

- dropdown: unity version
- checkboxes: the tools (file formats) you need (blender, etc)
- dropdown: GameCI version (feeding from docker releases list, auto picking the latest
- input text: docker username

result: a shell script that eases building and publishing customized docker images to docker hub.

Now this should be discussed and accepted in an issue before it gets developed to avoid wasting precious developer time 🙏

_Originally posted by @GabLeRoux in https://github.com/game-ci/docker/pull/181#issuecomment-1133642123_