Offer the container line to both shells, and name the short-path requirement

The container recipe carried a PowerShell backtick continuation in a snippet
whose next paragraph invites a POSIX shell to run it, where a backtick is not a
continuation. A continuation has to pick a shell, so the line does not have one.
The Dockerfile's own copy of the recipe still mounted one machine's checkout by
absolute path; it mounts whatever checkout is running it, like the other two.

And the requirement a newcomer meets by tripping over it: on win32-x64 the
harness needs the checkout somewhere short. MSBuild's file tracker gives out on
long paths, so a run from a deep temporary directory fails the build for every
mutant, the no-op control included. That is loud — the harness stops on a red
control and prints what the child said — but what the child says is MSBuild, not
the path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/quic-native-linux-dev.Dockerfile

@@ -9,8 +9,10 @@
 # runner that mutates it at all.
 #
 #   docker build -t cultlib-quic-native-dev -f scripts/quic-native-linux-dev.Dockerfile scripts
-#   docker run --rm --security-opt seccomp=unconfined -v F:\Projects\CultLib:/src -w /src \
-#       cultlib-quic-native-dev bash -lc "scripts/build-quic-native.sh && node scripts/mutate-cultmesh.mjs native"
+#   docker run --rm --security-opt seccomp=unconfined -v "${PWD}:/src" -w /src cultlib-quic-native-dev bash -lc "scripts/build-quic-native.sh && node scripts/mutate-cultmesh.mjs native"
+#
+# Both from the repository root, and the mount is that root wherever it is:
+# `${PWD}` in PowerShell, `$(pwd)` in a POSIX shell.
 #
 # `--security-opt seccomp=unconfined` is not optional and not caution. The
 # ThreadSanitizer configuration needs the process's address space where it