[ubuntu] feat(docker): authenticate using DockerHub credentials (#2548)

Ubuntu builds, by default, pull images anonymously from the
official DockerHub. DockerHub rate limits requests from
anonymous and unpaid accounts:
- https://docs.docker.com/docker-hub/download-rate-limit/

When those rate limits are reached the Packer build will
fail with an error indicated the rate limit has been
reached.

Add support for providing credentials for a paid
account that can be used by the Docker setup steps to
authenticate when pulling images from DockerHub, thereby
avoiding the rate limit.

Related to #2094

images/linux/scripts/installers/docker-moby.sh

@@ -28,10 +28,21 @@ systemctl is-enabled --quiet docker.service || systemctl enable docker.service
 sleep 10
 docker info
 
+# If credentials are provided, attempt to log into Docker Hub
+# with a paid account to avoid Docker Hub's rate limit.
+if [ "${DOCKERHUB_LOGIN}" ] && [ "${DOCKERHUB_PASSWORD}" ]; then
+  docker login --username "${DOCKERHUB_LOGIN}" --password "${DOCKERHUB_PASSWORD}"
+fi
+
 # Pull images
 images=$(get_toolset_value '.docker.images[]')
 for image in $images; do
     docker pull "$image"
 done
 
-invoke_tests "Tools" "Docker"
+# Always attempt to logout so we do not leave our credentials on the built
+# image. Logout _should_ return a zero exit code even if no credentials were
+# stored from earlier.
+docker logout
+
+invoke_tests "Tools" "Docker"