Add docker creds (#2129)

Author: anchitj

.semaphore/semaphore.yml

@@ -115,6 +115,9 @@ blocks:
       agent:
         machine:
           type: s1-prod-ubuntu20-04-amd64-2
+      prologue:
+        commands:
+          - docker login --username $DOCKERHUB_USER --password $DOCKERHUB_APIKEY
       jobs:
         - name: 'Build and test'
           commands:
@@ -128,6 +131,9 @@ blocks:
       agent:
         machine:
           type: s1-prod-ubuntu20-04-amd64-2
+      prologue:
+        commands:
+          - docker login --username $DOCKERHUB_USER --password $DOCKERHUB_APIKEY
       jobs:
         - name: 'Build and test'
           commands: