Encrypt DOCKER_PASS for gatsbyjs/gatsby (#4291)
Encrypt DOCKER_PASS for gatsbyjs/gatsby (#4291)

Travis' encrypted env vars are tied to individual repos. This env var
was originally encrypted through my forked copy of the Gatsby repo. The
test builds on my fork worked, but would then fail in the main Gatsby
repo.
gatsbyjs/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,gatsbyjs/gatsby,0x80/gatsby,gatsbyjs/gatsby,mingaldrichgan/gatsby,gatsbyjs/gatsby,ChristopherBiscardi/gatsby,fk/gatsby,mingaldrichgan/gatsby,0x80/gatsby,fk/gatsby,ChristopherBiscardi/gatsby,0x80/gatsby,ChristopherBiscardi/gatsby