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