feat: add devcontainer support

This should make it possible to run tests locally in a lighter way than pulling up a full VM.

.devcontainer/Dockerfile

@@ -0,0 +1,20 @@
+FROM dokku/dokku:latest
+
+RUN apt-get update
+RUN apt-get install --no-install-recommends -y build-essential && \
+  apt-get clean autoclean && \
+  apt-get autoremove --yes && \
+  rm -rf /var/lib/apt/lists/* && \
+  mkdir -p /mnt/dokku/home/dokku /mnt/dokku/var/lib/dokku/config /mnt/dokku/var/lib/dokku/data /mnt/dokku/var/lib/dokku/services && \
+  chown -R dokku:dokku /mnt/dokku/home/dokku /mnt/dokku/var/lib/dokku/config /mnt/dokku/var/lib/dokku/data /mnt/dokku/var/lib/dokku/services && \
+  echo "dokku.me" > /home/dokku/VHOST
+
+COPY . .
+
+RUN source /tmp/config && \
+  echo "export ${PLUGIN_DISABLE_PULL_VARIABLE}=true" > /tmp/.env && \
+  echo "export PLUGIN_NAME=${PLUGIN_COMMAND_PREFIX}" >> /tmp/.env
+
+RUN source /tmp/.env && \
+  dokku plugin:install file:///tmp --name $PLUGIN_NAME && \
+  make ci-dependencies