Resolve "Create a smaller deployment image"

Decreased the size of app container from 700Mb to 38Mb. By using linux alpine image it could probably be decreased even more (~10Mb), but in alpine image go app complains about some binaries it can't exec. For this demo I won't like bother with investigation.

Also I didn't use dind to build inside another container, just build it inside first docker.

Closes #4 (closed)