fixes to z-docker-opam example

[mostafatouny]

Modifications to Dockerfile fixes the following

json argument and casing.

missing os package gmp-dev

/usr/bin not found.

I lost the screenshot ;).

it is recommended to execute binaries using sh or ash in alpine; it has no bash out of the box.

COPY is more readable over ADD.

hello-opam.opam is not in the repo. probably a typo of z-docker-opam.opam file.

Docker compose is mentioned in another example. So we don't use it here.