[Docker] make docker image more lightweight

[IonBazan]

We don't need to pull the repo every time we build since the file should be in current build context. This should produce less layers than before. Note that yarn install is now executed before copying the sources to make use of build cache and build even faster!

[IonBazan]

Pinging @tokkenno for review :smiley:

[tokkenno]

@IonBazan Yes, I wrote the Dockerfile for my personal use and then decide do the pull request.. can be optimized.

Now I'm in the work, later when I get home, I will do some optimizations and will add some instructions to the README, but your code looks well.

I don't have permission to do the merge, @meepen has to do it.