feat(docker): refined Dockerfile

[Akagi201]

Refined the Dockerfile scripts.

[rphmeier]

cc @chevdor

[chevdor]

Thanks for the PR @Akagi201.
I wanted to have the dockerfile as independant as possible but your change will likely make it more maintainable.

[chevdor]

Not sure we need the ./init.sh (nor its content) at all actually. So please hold on before merging until I am done with a few tests.

[Akagi201]

@chevdor I post this PR to let the docker build process be the same as the manually build process. I think it is important for consistency.

When build manually, just run

./init.sh
./build.sh
cargo build

so the Dockerfile should be the same process.

I have tested it and you may check it twice.

[chevdor]

What I am trying to say is that ./init.sh and ./build.sh are not required in the build process.
You can have a look at #463 where I removed them entirely. It makes the image smaller and faster to build while having also less moving parts.

[Akagi201]

I agree with two stage docker build. It is also good for isolating the source code and binary. And I wrote one for my own use. https://github.com/Akagi201/dockerfile/blob/master/polkadot/Dockerfile

I am not sure the need for the init.sh and build.sh script. If they are not required, you may close this PR.

[chevdor]

What is the size of the image you get in the end with your 2 stages image?

[chevdor]

@Akagi201 Thanks again for this PR, I will close it as agreed since the scripts are definitely not required.