Tell CircleCI to build Docker images from `main` (#9906)

The `only` field takes a string or list of strings per the Circle docs:
https://circleci.com/docs/2.0/configuration-reference/#branches

Signed-off-by: Dan Callahan <danc@element.io>

.circleci/config.yml

@@ -41,7 +41,7 @@ workflows:
       - dockerhubuploadlatest:
           filters:
             branches:
-              only: master
+              only: [ master, main ]
 
 commands:
   docker_prepare:

changelog.d/9906.misc

@@ -0,0 +1 @@
+Tell CircleCI to build Docker images from `main` branch.