Building the tools image fails on Mac silicon

[chrsoo]

When following the instructions on CONTRIBUTING.md:

# change directory into the rust-fitsio root directory
docker build -t <tag> .
docker run --rm -it -v $(pwd):/project <tag> bash

... the docker build fails with the following message on a Mac with Apple silicon (M1):

➜  rust-fitsio git:(main) ✗ docker build -t fitsio .
[+] Building 165.4s (7/9)
...
 => ERROR [3/5] RUN rustup update &&     rustup install stable &&     rustup install nightly &&     rustup component add clippy --toolchain stable-x8  163.6s
------
 > [3/5] RUN rustup update &&     rustup install stable &&     rustup install nightly &&     rustup component add clippy --toolchain stable-x86_64-unknown-linux-gnu:
...
#6 163.6 error: toolchain 'stable-x86_64-unknown-linux-gnu' is not installed

It would appear that the Dockerfile (or build command) assumes an Intel-based Mac.

[chrsoo]

The Dockerfile explicitly refers to the x86_64 toolchain, preventing the Dockerfile to work on an arm-based machine:

RUN rustup update && \
    rustup install stable && \
    rustup install nightly && \
    rustup component add clippy --toolchain stable-x86_64-unknown-linux-gnu

However, the clippy documentation states that clippy is included in rustup per default, making this dependency unnecessary. Removing it makes the Dockerfile build on Mac and clippy still runs as expected:

➜  rust-fitsio git:(main) ✗ docker run --rm -it -v $(pwd):/project fitsio bash
root@7fd0895cfd46:/project# cargo clippy
info: syncing channel updates for 'beta-aarch64-unknown-linux-gnu'
info: latest update on 2024-12-28, rust version 1.84.0-beta.5 (0857a8e32 2024-12-27)
info: downloading component 'cargo'
info: downloading component 'clippy'
info: downloading component 'rust-analyzer'
info: downloading component 'rust-docs'
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: downloading component 'rustfmt'
info: installing component 'cargo'
info: installing component 'clippy'
info: installing component 'rust-analyzer'
info: installing component 'rust-docs'
info: installing component 'rust-std'
info: installing component 'rustc'
info: installing component 'rustfmt'
    Updating crates.io index
...

[chrsoo]

PR in #376