refactor: copy code into initial base image via docker build

The previous method used an ephemeral container, which was slightly more convoluted and required cleanup. The new method also allows us to set an initial WORKDIR, which is useful for ensuring 'docker exec' works as expected.

plugins/builder-herokuish/dockerfiles/copy-source.Dockerfile

@@ -0,0 +1,7 @@
+ARG APP_IMAGE
+FROM $APP_IMAGE
+
+ARG DOKKU_APP_USER herokuishuser
+RUN USER=$DOKKU_APP_USER /exec true
+COPY --chown=$DOKKU_APP_USER . /app
+WORKDIR /app