Add observable-builder user and use /project

Adds an observable-builder user with a home directory of /project. This
uses 8000 as the uid since that's not likely to collide with any users
on the container runtime host machines.

Dockerfile

@@ -125,3 +125,6 @@ COPY --from=python . .
 COPY --from=r . .
 COPY --from=duckdb . .
 COPY --from=rust . .
+RUN mkdir /project && useradd -m -d /project -u 8000 observable-builder && chown 8000:8000 /project
+WORKDIR /project
+USER 8000:8000