Automatically push new Docker images

[holly-hacker]

There should be some automated system to publish new Docker images for new stable releases of Rust, to avoid having to wait for an unresponsive maintainer.

Should resolve #80, #83, #86, etc.

[mbergkvist]

@holly-hacker I've moved to use AWS CDK to build and deploy Rust lambdas. See https://github.com/mbergkvist/rust-lambda-cdk for a sample project that automates build and deploy process.