Set the work directory for the nettest image

subctl expects "cat version" to work without changing directories, so
the work directory needs to be set to /app.

Signed-off-by: Stephen Kitt <skitt@redhat.com>

package/Dockerfile.nettest

@@ -16,6 +16,8 @@ ARG TARGETPLATFORM
 ARG VERSION
 ENV PATH="/app:$PATH"
 
+WORKDIR /app
+
 COPY --from=base /output/nettest /
 
 COPY scripts/nettest/* /app/