revert Dockerfile back to defining an entrypoint (#101)

Now that we are back to only 1 manager binary in the Dockerfile,
we can use entrypoint instead of command to lock down what is run.

Author: dgrove-oss

Dockerfile

@@ -31,4 +31,4 @@ WORKDIR /
 COPY --from=builder /workspace/manager .
 USER 65532:65532
 
-CMD ["/manager"]
+ENTRYPOINT ["/manager"]