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_create.bats

@@ -4,6 +4,7 @@ load test_helper
 @test "($PLUGIN_COMMAND_PREFIX:create) success" {
   run dokku "$PLUGIN_COMMAND_PREFIX:create" l
   assert_contains "${lines[*]}" "container created: l"
+  dokku --force "$PLUGIN_COMMAND_PREFIX:destroy" l
 }
 
 @test "($PLUGIN_COMMAND_PREFIX:create) error when there are no arguments" {