CARGO_HOME can't be mounted read-only when building with `--frozen`

[brson]

I have done cargo fetch and cargo generate-lockfile. Now i'm trying to run cargo build in a container, where CARGO_HOME is mounted read-only, assuming nothing in there needs to be touched. I get this error though:

101 ec2-user@ip-10-254-206-7:~/dev/cargobomb$ docker run -it -v `pwd`/dockertest:/test -v `pwd`/work/cargo-home:/cargo-home:ro -v `pwd`/work/rustup-home:/rustup-home:ro -v `pwd`/work/target-dirs/default:/target cargobomb cargo build --verbose
error: unable to get packages from source

Caused by:
  failed to open: /cargo-home/registry/cache/github.com-1ecc6299db9ec823/fake-simd-0.1.0.crate

Caused by:
  Read-only file system (os error 30)