slim down

[softprops]

there is a lot that rustup install pulls in that we don't need

softprops/lambda-rust 0.2.0-rust-1.31.0 ace6af5279eb 3 days ago 2.48GB
lambci/lambda build-provided 3d4454c76ff7 13 days ago 1.68GB

install docker-diff then run the following and do some analysis what could be trimmed

$ docker-diff lambci/lambda:build-provided softprops/lambda-rust:0.2.0-rust-1.31.0

[softprops]

https://blog.rust-lang.org/2019/10/15/Rustup-1.20.0.html this should be ready to cook now

[softprops]

before

docker history softprops/lambda-rust:0.2.4-rust-1.38.0 | grep '1.38.0 /bin/sh -c curl'
<missing> 9 days ago |1 RUST_VERSION=1.38.0 /bin/sh -c curl https… 1.09GB

after

$ docker history softprops/lambda-rust:0.2.5-rust-1.38.0 | grep '1.38.0 /bin/sh -c curl'
e4dd564e5670 2 minutes ago |1 RUST_VERSION=1.38.0 /bin/sh -c curl --pro… 813MB

$ docker images | grep rust-1.38.0
softprops/lambda-rust 0.2.5-rust-1.38.0 b48ac4b7955e 9 minutes ago 2.66GB
softprops/lambda-rust 0.2.4-rust-1.38.0 fa7b84ae56e1 9 days ago 2.94GB