Update Docker repo to download.docker.com

[jongiddy]

High-level description (required)

Move the Docker repo from dockerproject.org to download.docker.com as indicated at https://www.docker.com/blog/changes-dockerproject-org-apt-yum-repositories

Corresponding tickets (required)

These JIRA ticket(s) must be updated (ideally closed) in the moment this PR lands:

• DCOS_OSS-5957 minidcos vagrant fails with yum.dockerproject.org not found

Related tickets (optional)

Other tickets related to this change

• DCOS- Foo the Bar so it stops Bazzing.

Related PRs (optional)

Is this change going to be propagated up into dcos-integration-tests in dcos/dcos or another repo? Does this change require changes in dcos-test-utils? Link the corresponding PRs here:

Checklist for all PRs

• Included a test which will fail if code is reverted but test is not. If there is no test please explain here:
• Added or updated any relevant documentation

Integration tests were run and

• AWS Cloudformation Simple (link to job: )
• Azure Resource Manager (link to job: )
• Onprem-AWS (link to job: )
• Onprem-GCE (link to job: )

PLEASE FILL IN THE TEMPLATE ABOVE / DO NOT REMOVE ANY SECTIONS ABOVE THIS LINE

Instructions and review process

What is the review process and when will my changes land?

All PRs should have two approved pull request reviews.

Reviewers should be:

• Developers who understand the code being modified.
• Developers responsible for code that interacts with or depends on the code being modified.

It is best to proactively ask for 2 reviews by @mentioning the candidate reviewers in the PR comments area. The responsibility is on the developer submitting the PR to follow-up with reviewers and make sure a PR is reviewed in a timely manner.