Add docker-rust image

ci/docker-rust/Dockerfile

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+FROM rust
+
+RUN apt update && \
+    apt-get install apt-transport-https && \
+    echo deb https://apt.buildkite.com/buildkite-agent stable main > /etc/apt/sources.list.d/buildkite-agent.list && \
+    apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv-keys 32A37959C2FA5C3C99EFBC32A79206696452D198 && \
+    apt update && \
+    apt install -y \
+      buildkite-agent \
+      rsync \
+      sudo \
+      && \
+    rm -rf /var/lib/apt/lists/*

ci/docker-rust/README.md

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+Docker image containing rust and some preinstalled packages used in CI.
+
+This image may be manually updated by running `./build.sh` if you are a member
+of the [Solana Labs](https://hub.docker.com/u/solanalabs/) Docker Hub
+organization, but it is also automatically updated periodically by
+[this automation](https://buildkite.com/solana-labs/solana-ci-docker-rust).

ci/docker-rust/build.sh

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+#!/bin/bash -ex
+
+cd "$(dirname "$0")"
+
+docker build -t solanalabs/rust .
+docker push solanalabs/rust