Use fully qualified image references in Tekton tests (knative#701)

test/test_images/helloworld/Dockerfile

@@ -1,7 +1,7 @@
 # Use the official Golang image to create a build artifact.
 # This is based on Debian and sets the GOPATH to /go.
 # https://hub.docker.com/_/golang
-FROM golang:1.13 as builder
+FROM docker.io/library/golang:1.13 as builder
 
 # Create and change to the app directory.
 WORKDIR /app
@@ -15,7 +15,7 @@ RUN CGO_ENABLED=0 GOOS=linux go build -v -o server
 # Use the official Alpine image for a lean production container.
 # https://hub.docker.com/_/alpine
 # https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
-FROM alpine:3
+FROM docker.io/library/alpine:3
 RUN apk add --no-cache ca-certificates
 
 # Copy the binary to the production image from the builder stage.