feat: Real docker-based testing

This pull request switches testing to use an actual docker daemon, vs mocking everything out.

It may also catch actual breaking issues in our tests, which is great!

tests/service_unlink.bats

@@ -2,13 +2,13 @@
 load test_helper
 
 setup() {
-  dokku apps:create my_app >&2
-  dokku "$PLUGIN_COMMAND_PREFIX:create" l >&2
+  dokku apps:create my_app
+  dokku "$PLUGIN_COMMAND_PREFIX:create" l
 }
 
 teardown() {
-  dokku --force "$PLUGIN_COMMAND_PREFIX:destroy" l >&2
-  rm -rf "$DOKKU_ROOT/my_app"
+  dokku --force "$PLUGIN_COMMAND_PREFIX:destroy" l
+  dokku --force apps:destroy my_app
 }
 
 @test "($PLUGIN_COMMAND_PREFIX:unlink) error when there are no arguments" {