dev: add file (for linting) and nano (for in-container editing) to devcontainer

.devcontainer/Dockerfile

@@ -1,7 +1,7 @@
 FROM dokku/dokku:latest
 
 RUN apt-get update
-RUN apt-get install --no-install-recommends -y build-essential && \
+RUN apt-get install --no-install-recommends -y build-essential file nano && \
   apt-get clean autoclean && \
   apt-get autoremove --yes && \
   rm -rf /var/lib/apt/lists/* && \