Add dockerfile for gitpod rust contract build

[ailisp]

Associated Issue: #235

Summary of Changes

• Add a dockerfile for used by gitpod to build rust smart contract

Test Plan

Not testable now but after gitpod is installed. It should be able to build a rust smart contract. Reference: https://www.gitpod.io/docs/42_config_docker/ https://github.com/gitpod-io/workspace-images/blob/master/full/Dockerfile

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[vgrichina]

@ailisp let's create separate repo for GitPod stuff

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[ailisp]

Sounds good!