fix(build-system): Login to dockerhub (#4716)

This regressed in
#2353 (Charlie's PR
but mea culpa)

build-system/scripts/build

@@ -53,6 +53,8 @@ function try_fetch_image() {
 
 # Login to ECR and ensure repository exists.
 retry ensure_repo $REPOSITORY $ECR_REGION refresh_lifecycle
+# Login to dockerhub.
+retry dockerhub_login
 
 CONTENT_HASH=$(calculate_content_hash $REPOSITORY)
 echo "Content hash: $CONTENT_HASH"