use cargo-install to install josh-proxy

src/tools/miri/CONTRIBUTING.md

@@ -233,14 +233,14 @@ You can also directly run Miri on a Rust source file:
 ## Advanced topic: Syncing with the rustc repo
 
 We use the [`josh` proxy](https://github.com/josh-project/josh) to transmit changes between the
-rustc and Miri repositories. The eaisest way to run josh is via docker:
+rustc and Miri repositories.
 
 ```sh
-docker pull joshproject/josh-proxy:latest
-docker run -it -p 8000:8000 -e JOSH_REMOTE=https://github.com -e JOSH_EXTRA_OPTS=--no-background -v josh-vol:/data/git joshproject/josh-proxy:latest
+cargo +stable install josh-proxy --git https://github.com/josh-project/josh --tag r22.12.06
+josh-proxy --local=$HOME/.cache/josh --remote=https://github.com --no-background
 ```
 
-This sets up a local volume `josh-vol` for josh's cache.
+This uses a directory `$HOME/.cache/josh` as a cache, to speed up repeated pulling/pushing.
 
 ### Importing changes from the rustc repo