fix: only require the docker bin

Docker has this thing where they rebrand the debian package name once a year, and as YUM doesn't resolve anything like a "Provides" field, this is the best we can do (unless they all required the same metapackage).

Note that now we can't ensure a specific minor docker version via RPM, though I feel as though if you choose a crappy OS/package manager and we fail you, thats on you, not Dokku.

Closes #2732

rpm.mk

@@ -97,7 +97,7 @@ endif
 		--depends 'man-db' \
 		--depends 'sshcommand' \
 		--depends 'gliderlabs-sigil' \
-		--depends 'docker-engine >= 1.9.1 | docker-engine-cs >= 1.9.1 | docker-ce | docker-ee' \
+		--depends '/usr/bin/docker' \
 		--depends 'bind-utils' \
 		--depends 'nginx' \
 		--depends 'plugn' \