Document Docker Desktop single-file bind-mount gotcha

On Docker Desktop (macOS/Windows), single-file bind-mounts can
silently stop propagating host edits — the file gets materialized
onto the VM's ext4 disk and reused forever. This affects anyone who
uncomments the ~/.bash_aliases or ~/.inputrc mount lines.

Add a caveat note in README.md's 'Overriding the defaults / Option A'
section with the verification command and the directory-mount
workaround. Add a matching inline NOTE comment in docker-compose.yml
above the commented mount lines. Linux hosts are unaffected.

docker-compose.yml

@@ -64,6 +64,11 @@ services:
       # Optional: override baked shell defaults with your host's rc files.
       # The image ships sensible defaults (history tuning, prefix-search on
       # Up/Down arrows, fzf/zoxide integration). Uncomment to use your own:
+      #
+      # NOTE: On Docker Desktop (macOS/Windows), single-file bind-mounts can
+      # silently stop propagating host edits — the file gets materialized onto
+      # the VM disk. If host changes don't appear in the container, see the
+      # "Shell defaults" section in README.md. Linux hosts are unaffected.
       # - ~/.bash_aliases:/home/developer/.bash_aliases:ro
       # - ~/.inputrc:/home/developer/.inputrc:ro