Don't overwrite local docker installs on linux

If a user has installed docker via other means, checking for docker.io
will cause apt-get to install over it and break the existing docker
installation. Since docker-compose will install docker.io recursively,
this is safer for users that have things configured already.

Change-Id: I6c45c70fbcdcdb8597310372fa331d764b1dd859
Reviewed-on: https://gerrit.instructure.com/158944
Reviewed-by: Omar Khan <okhan@instructure.com>
Product-Review: Steve Kacsmark <skacsmark@instructure.com>
QA-Review: Steve Kacsmark <skacsmark@instructure.com>
Tested-by: Jenkins

script/docker_dev_setup.sh

@@ -41,7 +41,7 @@ if [[ $OS == 'Darwin' ]]; then
   dependencies='docker docker-machine docker-compose dinghy'
 elif [[ $OS == 'Linux' ]]; then
   install='sudo apt-get update && sudo apt-get install -y'
-  dependencies='docker.io docker-compose'
+  dependencies='docker-compose'
 else
   echo 'This script only supports MacOS and Linux :('
   exit 1