chore(ci): fetch docker username and password and update docker registry auth (#1264)

Set docker username and fetch the password before running docker tests

### Test plan
CI

Author: burmudar

.buildkite/vagrant-run.sh

@@ -26,6 +26,8 @@ if [ ! -f ${KEY_PATH} ]; then
   gcloud secrets versions access latest --secret=e2e-builder-sa-key --quiet --project=sourcegraph-ci > "${KEY_PATH}"
 fi
 export GOOGLE_JSON_KEY_LOCATION="${KEY_PATH}"
+export DOCKER_USERNAME=sourcegraph
+export DOCKER_PASSWORD="$(gcloud secrets versions access latest --secret=DOCKER_PASSWORD_BUILDKITE_AGENT --quiet --project=sourcegraph-ci)"
 
 echo --- ":vagrant: starting box $box"
 vagrant up "$box" --provider=google || exit_code=$?

test/smoke-test.sh

@@ -2,11 +2,13 @@
 set -euxfo pipefail
 
 configure_docker() {
+  set +x
   if [ -n "${DOCKER_USERNAME}" ] && [ -n "${DOCKER_PASSWORD}" ]; then
     docker login -u "${DOCKER_USERNAME}" --password-stdin <<<"$DOCKER_PASSWORD"
   fi
+  set -x
   gcloud auth configure-docker
-  gcloud auth configure-docker us-central1-docker.pkg.dev
+  gcloud auth configure-docker us-central1-docker.pkg.dev,us-docker.pkg.dev
 }
 
 deploy_sourcegraph() {