Dockerfile optimization (#2598)

* metadata for docker image

* metadata for docker image

substrate/scripts/docker/Dockerfile

@@ -1,25 +1,40 @@
 FROM debian:stretch-slim
-LABEL maintainer "devops-team@parity.io"
-LABEL description="Substrate: The platform for blockchain innovators"
 
+# metadata
+ARG VCS_REF
+ARG BUILD_DATE
+
+LABEL io.parity.image.authors="devops-team@parity.io" \
+	io.parity.image.vendor="Parity Technologies" \
+	io.parity.image.title="parity/substrate" \
+	io.parity.image.description="Substrate: The platform for blockchain innovators." \
+	io.parity.image.source="https://github.com/paritytech/substrate/blob/master/scripts/docker/Dockerfile" \
+	io.parity.image.revision="${VCS_REF}" \
+	io.parity.image.created="${BUILD_DATE}" \
+	io.parity.image.documentation="https://wiki.parity.io/Parity-Substrate"
+
+# show backtraces
+ENV RUST_BACKTRACE 1
+
+# install tools and dependencies
 RUN apt-get update && \
-    DEBIAN_FRONTEND=noninteractive apt-get upgrade -y && \
-    DEBIAN_FRONTEND=noninteractive apt-get install -y \
-      libssl1.1 \
-      ca-certificates \
-      curl && \
-    apt-get autoremove -y && \
-    apt-get clean && \
-    find /var/lib/apt/lists/ -type f -not -name lock -delete
+	DEBIAN_FRONTEND=noninteractive apt-get upgrade -y && \
+	DEBIAN_FRONTEND=noninteractive apt-get install -y \
+		libssl1.1 \
+		ca-certificates \
+		curl && \
+# apt cleanup
+	apt-get autoremove -y && \
+	apt-get clean && \
+	find /var/lib/apt/lists/ -type f -not -name lock -delete; \
+# add user
+	useradd -m -u 1000 -U -s /bin/sh -d /substrate substrate
 
+# add substrate binary to docker image
 COPY ./substrate /usr/local/bin
 
-
-RUN useradd -m -u 1000 -U -s /bin/sh -d /substrate substrate
 USER substrate
 
-ENV RUST_BACKTRACE 1
-
 # check if executable works in this container
 RUN /usr/local/bin/substrate --version