Use docker mirror in CI (#82)

diff --git a/.circleci/config.yml b/.circleci/config.yml
index 7289c6b..2219511 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -2,7 +2,7 @@
 
 references:
   images:
-    go: &GOLANG_IMAGE circleci/golang:latest
+    go: &GOLANG_IMAGE docker.mirror.hashicorp.services/circleci/golang:1.15.3
   environments:
     tmp: &TEST_RESULTS_PATH /tmp/test-results # path to where test results are saved