web: Reduce docker build IO

- binaryen xz is no longer stored on disk, instead being piped directly to tar
- only extract wasm-opt (the only file we're interested in)
- locate wasm-opt using wildcard, instead of keeping path in sync with version number.

we're doing the same with sh.rustup.rs below.

web/docker/Dockerfile

@@ -6,9 +6,9 @@ FROM node:22
 
 # Installing wasm-opt from GitHub:
 # Keep the version number in sync with the ones in the Actions workflows!
-RUN wget --progress=:giga https://github.com/WebAssembly/binaryen/releases/download/version_121/binaryen-version_121-x86_64-linux.tar.gz
-RUN tar xzf binaryen-version_121-x86_64-linux.tar.gz binaryen-version_121/bin/wasm-opt
-RUN mv binaryen-version_121/bin/wasm-opt /usr/local/bin
+RUN wget --progress=:giga https://github.com/WebAssembly/binaryen/releases/download/version_121/binaryen-version_121-x86_64-linux.tar.gz -O- | \
+    tar xzf - --wildcards "*wasm-opt" --strip-components=2 && \
+    mv wasm-opt /usr/local/bin
 
 # Installing Rust using rustup:
 RUN wget 'https://sh.rustup.rs' --quiet -O- | sh -s -- -y --profile minimal --target wasm32-unknown-unknown