tests: run mvdan/shfmt on test runs

While I do not agree with _every_ style change, this will force Dokku to have consistent formatting across all shell scripts, which is arguably a Good Thing™.

The command used to reprocess everything is:

```shell
shfmt -l -bn -ci -i 2 -w .
```

plugins/config/docker-args-deploy

@@ -1,5 +1,6 @@
 #!/usr/bin/env bash
-set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
 source "$PLUGIN_AVAILABLE_PATH/config/functions"
 
@@ -9,7 +10,7 @@ config_docker_args() {
   local ENV_ARGS IMAGE STDIN trigger
 
   IMAGE=$(get_deploying_app_image_name "$APP" "$IMAGE_TAG")
-  STDIN=$(cat);
+  STDIN=$(cat)
   trigger="$0 config_docker_args"
   verify_app_name "$APP"