Dockerfile to build container used by TaskCluster

[edunham]

TC pulls the container from https://hub.docker.com/r/servobrowser/servo-linux-dev/

Perms to upload there are managed with other infra secrets

This is as the Dockerfile stood when I last uploaded; future work includes
automating the build and push upon Dockerfile changes landing in this repo.

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• ./mach build -d does not report any errors
• ./mach test-tidy does not report any errors
• These changes address the issue that edunham is embarrassed to send a "we're changing a thing" email without the "and you can change it too" step, which requires the Dockerfile that generated the container that TaskCluster pulls from to live in a sensible public place

• There are tests for these changes OR
• These changes do not require tests because _____
• these changes are prerequisite to adding more better tests

r? @aneeshusa

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This change is 

[highfive]

Heads up! This PR modifies the following files:

• @aneeshusa: etc/ci/Dockerfile

[aneeshusa]

@edunham Instead of manually listing out the (os-specific) instructions in the Dockerfile (apt-get, pip install), I would prefer to leverage our existing investment in Salt for defining build requirements.
How do you feel about servo/saltfs#768 instead?
Having this live in saltfs for now also means we can auto-build the Dockerfile on each saltfs change via TaskCluster, instead of having to come and manually update this Dockerfile later for any changes.

[aneeshusa]

@edunham @metajack @larsbergstrom could you please chime in on this or the other PR? It sounds like the Webrender folks are having a really great experience with TC so far, so I'd love to make progress on the Servo side!

[larsbergstrom]

@edunham ping :-)

[edunham]

This is currently wrong because we switched to using rustup on every build anyways...

[aneeshusa]

Closing as servo/saltfs#768 has been merged.