[infer][PR] Improvements in docker container generation
[infer][PR] Improvements in docker container generation

Summary:
Using "opam init" in non interactive mode, in order to avoid compilation failures. Otherwise, It will wait for user input until a timeout, that will cause the compilation process to fail due to an OPAM error.

Also the Dockerfile has been modified in order to use [multistage builds](https://docs.docker.com/develop/develop-images/multistage-build/), obtaining a cleaner Docker image.
Pull Request resolved: https://github.com/facebook/infer/pull/1102

Reviewed By: jvillard

Differential Revision: D15430898

Pulled By: mbouaziz

fbshipit-source-id: 0718163bb

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