Add full documentation for image

[AbcSxyZ]

Create a draft for the image documentation. Can be the base to use for the readme available both from GitHub and from DockerHub.

You can have an overview of the result on the following Docker Hub repository.

Inspired by documentation of official Docker Hub images.

I want also to discuss (at least) the following line:

docker exec (--user dogecoin ??) dogecoin-node dogecoin-cli help

When using docker exec, commands are launched as root. Idk if we specify to use a user ? We may specify to use the cli a different way, by creating another a new container for example.

Edit: Executable launched with docker exec are actually under dogecoin user rights, because I've setuid binaries. docker exec start as root and then switch to user.

[AbcSxyZ]

I think it's something we can start to consider. I did some changes about docker exec, executable are setuid are running under dogecoin user in any case.

I created a separate IMAGE_README.md for the image documentation. I think this file could be used to explain image on Docker Hub.

This way we could have 2 README, with the main README of this repository explaining how to build manually images, run tests, develop & contribute, tools doc, etc, focusing more on image development. With the main README which still link to IMAGE_README.md to explain usage for those who use this path.

[patricklodder]

Some minor comments inline. I would suggest we get this ready, but not publish it before we are capable of publishing the images described here.

[AbcSxyZ]

In my opinion, it would be interesting to add this documentation when it's ready. For now, it's stated in big that the project is under development (we can add it at the top of this doc too), if someone come here he could be aware of this and still try to play with the image. Maybe some potential contributor could come and have something a bit documented to try.

If someone follows the doc, he will be quickly stopped by not being able to download the image from dogecoin/dogecoin. Adventurous people could slowly start to tests it, but it's still not easily available with Docker Hub nor Dogecoin Core repo, it limits widely the scope of people who will find it and think it's ready by mistake.

For me we should even add a link into the main repo, saying it's not finished, but a beginning of a Dockerfile is testable (but definitively unstable, available for development only). It would facilitate people participation.