Cannot install mdbook plugins

[GMartinez-Sisti]

Hi, after this PR our custom builds started to fail with:

144.9 error: failed to compile `mdbook-external-links v0.1.2`, intermediate artifacts can be found at `/tmp/cargo-installjrZlXk`.
144.9 To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.
144.9 
144.9 Caused by:
144.9   rustc 1.78.0 is not supported by the following packages:
144.9     litemap@0.7.5 requires rustc 1.81
144.9     zerofrom@0.1.6 requires rustc 1.81
144.9   Try re-running `cargo install` with `--locked`
144.9      Summary Successfully installed mdbook-mermaid! Failed to install mdbook-external-links (see error(s) above).
144.9 error: some crates failed to install

We use a simple dockerfile with:

FROM peaceiris/mdbook:v0.4.40-rust

SHELL ["/bin/sh", "-exc"]

RUN apk add musl-dev && \
    rustup update stable && \
    rustup default stable && \
    cargo install \
        mdbook-external-links \
        mdbook-mermaid

To fix this I added an update to the latest stable rust version:

FROM peaceiris/mdbook:v0.4.40-rust

SHELL ["/bin/sh", "-exc"]

RUN apk add musl-dev && \
    rustup update stable && \
    rustup default stable && \
    cargo install \
        mdbook-external-links \
        mdbook-mermaid

Is this something we could add to the current build? Happy to send a PR.