Remove unused repo parts (#4271)

* Remove unused repo parts

* Update README

* cargo fmt

docker/Dockerfile

@@ -1,11 +0,0 @@
-ARG FROMTAG=latest
-FROM quay.io/nushell/nu-base:${FROMTAG} as base
-FROM ubuntu:18.04
-COPY --from=base /usr/local/bin/nu /usr/local/bin/nu
-ENV DEBIAN_FRONTEND noninteractive
-RUN apt-get update \
-    && apt-get install -y libssl-dev pkg-config \
-    && apt-get clean \
-    && rm -fr /var/lib/apt/lists/*
-ENTRYPOINT ["nu"]
-CMD ["-l", "info"]