Update NPM base image to 20.04 (#552)

npm/Dockerfile

@@ -1,5 +1,5 @@
 # Use a minimal image as a parent image
-FROM ubuntu:19.10
+FROM ubuntu:focal
 ARG NPM_BUILD_DIR
 
 # Install dependencies.
@@ -15,4 +15,4 @@ COPY $NPM_BUILD_DIR/azure-npm /usr/bin
 WORKDIR /usr/bin
 
 # Run the npm command by default when the container starts.
-ENTRYPOINT ["/usr/bin/azure-npm"]
+ENTRYPOINT ["/usr/bin/azure-npm"]