Make cargo new independent of workdir to be exact

The muslrust images seem to have a workdir of /volume as opposed to / in the
others so doing cargo new like this would create the folder in /volume/app.

docker/Dockerfile.j2

@@ -156,7 +156,7 @@ RUN apt-get update && apt-get install -y \
 
 {% endif %}
 # Creates a dummy project used to grab dependencies
-RUN USER=root cargo new --bin app
+RUN USER=root cargo new --bin /app
 WORKDIR /app
 
 # Copies over *only* your manifests and build files