Update Dockerfile

This will preserve setups that were configured to run the old way without breaking them and new configs can be setup "correctly" without issue.

Dockerfile

@@ -10,4 +10,7 @@ RUN apk update && apk add go git gcc musl-dev \
 FROM alpine:edge
 RUN apk --no-cache add ca-certificates mailcap
 COPY --from=builder /bin/matterbridge /bin/matterbridge
+RUN mkdir /conf
+RUN touch /conf/matterbridge.toml
+RUN ln -sf /matterbridge.toml /conf/matterbridge.toml
 ENTRYPOINT ["/bin/matterbridge", "-conf", "/conf/matterbridge.toml"]