Add file volume to the docker image

Dockerfile

@@ -15,22 +15,28 @@ RUN apk --no-cache add build-base git
 COPY . ${GOPATH}/src/code.vikunja.io/api
 WORKDIR ${GOPATH}/src/code.vikunja.io/api
 
-#Checkout version if set
+# Checkout version if set
 RUN if [ -n "${VIKUNJA_VERSION}" ]; then git checkout "${VIKUNJA_VERSION}"; fi \
  && make clean generate build
 
 ###################
 # The actual image
 # Note: I wanted to use the scratch image here, but unfortunatly the go-sqlite bindings require cgo and
-# for whatever reason, the container would not start when I compiled the image without cgo.
+# because of this, the container would not start when I compiled the image without cgo.
 FROM alpine:3.9
 LABEL maintainer="maintainers@vikunja.io"
 
 WORKDIR /app/vikunja/
 COPY --from=build-env /go/src/code.vikunja.io/api/vikunja .
-RUN chown nobody:nogroup -R /app/vikunja
+RUN adduser -S -D vikunja -h /app/vikunja -H \
+  && chown vikunja -R /app/vikunja
 ENV VIKUNJA_SERVICE_ROOTPATH=/app/vikunja/
 
-USER nobody:nogroup
+# Files permissions
+RUN mkdir /app/vikunja/files && \
+  chown -R vikunja /app/vikunja/files
+VOLUME /app/vikunja/files
+
+USER vikunja
 CMD ["/app/vikunja/vikunja"]
 EXPOSE 3456