This commit adds the known names of the rust-toolchain files to the
.dockerignore file. This has two reasons why it makes sense:

- The initial docker layer already has a set up rust toolchain that is
  sufficient to build the project. Thus, by providing a toolchain file,
  the toolchain would be installed again during docker build.
- Currently cargo-chef only copies the toolchain files during cooking
  but it gets not used during the building of the dependencies in the
  cook call, see
  LukeMathWalker/cargo-chef#271.
  With this in mind, currently the dependencies were actually build
  twice. Once with the installed toolchain from the image itself, and
  then in the actual cargo build call with the toolchain speciefied in
  the toolchain file. Building them twice resulted in timeouts when
  building the arm64 images as they are emulated using qemu, which is
  itself already slower than building natively.

Now one could argue, that as soon as the mentioned issue is solved using
the toolchain again would be fine. But then it would be still needed to
assemble the Dockerfile in a way that the toolchain is not build twice.
Because the current structure of the Dockerfile builds the toolchain
once in the cargo-chef prepare step and once during the cargo build step
(and would later build it during the cargo-chef cook instead of cargo
build).

With all this in mind using no toolchain file but instead just using
the sufficient rust installation from the base image makes sense.