feat: add support for VSCode Dev Containers

This cannot be used in codespaces due to the extra mount, but it does allow for local development.

.devcontainer/Dockerfile

@@ -0,0 +1,17 @@
+FROM dokku/dokku:latest
+
+RUN apt-get update
+RUN apt-get install --no-install-recommends -y build-essential file nano && \
+  apt-get install --no-install-recommends -y shellcheck xmlstarlet && \
+  apt-get clean autoclean && \
+  apt-get autoremove --yes && \
+  rm -rf /var/lib/apt/lists/*
+
+ADD https://raw.githubusercontent.com/dokku/dokku/master/tests/dhparam.pem /mnt/dokku/etc/nginx/dhparam.pem
+
+COPY .devcontainer/bin/ /usr/local/bin/
+COPY . .
+
+RUN make ci-dependencies
+
+ENV DOKKU_HOSTNAME=dokku.me