Check for docker in linux deploy_stack scripts

This commit updates the linux (both ARM and x86) deploy_stack scripts to
check for the availability of the Docker command and print the same
error returned by the PS1 script if not found.

Signed-off-by: John McCabe <john@johnmccabe.net>

deploy_stack.armhf.sh

@@ -1,4 +1,9 @@
 #!/bin/sh
 
+if ! [ -x "$(command -v docker)" ]; then
+  echo 'Unable to find docker command, please install Docker (https://www.docker.com/) and retry' >&2
+  exit 1
+fi
+
 echo "Deploying stack"
 docker stack deploy func --compose-file docker-compose.armhf.yml