Create Dockerfile.ppc64le

[rpsene]

This commit adds a new Dockerfile for ppc64le which builds travis-worker binary and run it inside a Docker container.

What is the problem that this PR is trying to fix?

Adds support for building travis-worker for ppc64le within a Docker container

What approach did you choose and why?

Just replaced the base images with the ones related to ppc64le

How can you test this?

Run on Power (you can get free access to Power at https://minicloud.parqtec.unicamp.br/)

What feedback would you like, if any?