1. Go to Rustup components availability page
2. Find out latest version that supports rustfmt component (e.g. 2021-03-25)
3. Edit Dockerfile. Change RUST_VERSION to the target version e.g. nightly-2021-03-25.
4. Build and push:

docker build -t howardwu/snarkos-ci:2021-03-25 .
docker push howardwu/snarkos-ci:2021-03-25