message: docs: Add to instructions for building docker image
  I ran into a compilation error because I was missing the `x86_64-unknown-linux-musl` target. This adds instructions for installing that.
diff: @@ -176,6 +176,12 @@ $ ./k8-util/cluster/reset-kind.sh
  #### Build Fluvio CLI and docker image from source code
  ```
  +# Install the cross-compilation toolchain needed to compile the docker image (only required the first time)
  +# x86_64 (most computers):
  +$ rustup target add x86_64-unknown-linux-musl
  +# M1 Macs:
  +$ rustup target add aarch64-unknown-linux-musl
  +
  # This will build the Fluvio cli and then create a docker image
  $ make build-cli build_k8_image
  ```
  