Adds dockerfile for build image

[ianphil]

All Submissions:

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• [x] Have you followed the guidelines in our Contributing document?
• [x] Have you added an explanation of what your changes do and why you'd like us to include them?
• [x] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.
• [x] My code follows the code style of this project.
• [x] I ran lint checks locally prior to submission.
• [x] Have you checked to ensure there aren't other open Pull Requests for the same update/change?

What is the current behavior?

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Currently, all build dependencies have to be installed manually or by the pipeline using scripts.

Issue Number: #306

What is the new behavior?

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Dockerfile that contains all of the build dependencies that can be run in the custom build agent.

Does this introduce a breaking change?

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• [x] NO

Any relevant logs, error output, etc?

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(If it’s long, please paste to https://ghostbin.com/ and insert the link here.)

Other information

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To test:

1. Build image docker build -t cobaltbuilder .
2. Run image docker run -it cobaltbuilder bash
3. Test version of TF terraform version
4. Test version of go go version
5. Test version of az az version