Docker image containing rust and some preinstalled packages used in CI.

NOTE: Recreate rust-nightly docker image after this when updating the stable rust version! Both of docker images must be updated in tandem.

This image manually maintained:

1. Edit Dockerfile to match the desired rust version
2. Run docker login to enable pushing images to Docker Hub, if you're authorized.
3. Run ./build.sh to publish the new image, if you are a member of the Solana Labs Docker Hub organization.