fix install instruction

docs/docker/Dockerfile

@@ -1,16 +0,0 @@
-FROM debian:stable-slim AS nodejs-base
-RUN apt-get -q update && \
-    env DEBIAN_FRONTEND=noninteractive apt-get -y install git nodejs yarnpkg && \
-    apt-get clean && rm -fr /var/lib/apt/lists/*
-
-FROM nodejs-base AS build
-RUN yarnpkg global add --latest --production --silent http://gancio.org/latest.tgz && \
-    apt-get clean && rm -fr /var/lib/apt/lists/*
-
-FROM nodejs-base
-COPY --from=build /usr/local/share/.config/yarn/ /usr/local/share/.config/yarn/
-RUN ln -s ../share/.config/yarn/global/node_modules/.bin/gancio /usr/local/bin/gancio
-
-ENTRYPOINT ["/usr/local/bin/gancio"]
-
-