remove debian specific check in docker-dev-setup

Removing the apt-get check will allow the script to run on more Linux
distros.
This change also modifies the messaging and specifically state that
running this script on a non Debian distro may or may not work and is
not supported should that state change in the future.

refs DE-538
flag = none

test-plan:
- Ensure docker_dev_setup is successfull on Mac and Linux

Change-Id: I1e95b5dad2af4198d6e7007966b1653dcfe13bb4
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/260375
Reviewed-by: James Butters <jbutters@instructure.com>
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
QA-Review: Andrea Cirulli <andrea.cirulli@instructure.com>
Product-Review: Andrea Cirulli <andrea.cirulli@instructure.com>

script/docker_dev_setup.sh

@@ -64,10 +64,10 @@ DOCKER='y'
 if [[ $OS == 'Darwin' ]]; then
   #docker-compose is checked separately
   dependencies='docker docker-machine dinghy'
-elif [[ $OS == 'Linux' ]] && ! installed apt-get; then
-  echo 'This script only supports Debian-based Linux'
-  exit 1
 elif [[ $OS == 'Linux' ]]; then
+  if [ ! -f "/etc/debian_version" ]; then
+    echo "Running this script on a non Debian distro may or may not work and is not officially supported"
+  fi
   #when more dependencies get added, modify Linux install output below
   #docker-compose is checked separately
   dependencies='dory'