Jenkinsfile.public: specify main branch for docker build

Docker build defaults to the master branch, when specifying a git repo.
With this change, it should be smart enough to use the proper main branch.

Jenkinsfile.public

@@ -2,7 +2,7 @@ pipeline {
     agent {
         docker {
             label 'main'
-            image docker.build("storj-ci", "--pull https://github.com/storj/ci.git").id
+            image docker.build("storj-ci", "--pull git://github.com/storj/ci.git#main").id
             args '-u root:root --cap-add SYS_PTRACE -v "/tmp/gomod":/go/pkg/mod -v "/tmp/npm":/npm --tmpfs "/tmp:exec,mode=777"'
         }
     }