ci: fix dockerfile for debian build (#2194)

substrate/scripts/docker/Dockerfile

@@ -1,27 +1,28 @@
-FROM ubuntu:xenial
+FROM debian:stretch-slim
 LABEL maintainer "devops-team@parity.io"
 LABEL description="Substrate: The platform for blockchain innovators"
 
 RUN apt-get update && \
-    apt-get upgrade -y && \
+    DEBIAN_FRONTEND=noninteractive apt-get upgrade -y && \
     DEBIAN_FRONTEND=noninteractive apt-get install -y \
-		libssl1.0.0 \
-		ca-certificates \
-		curl && \
+      libssl1.1 \
+      ca-certificates \
+      curl && \
     apt-get autoremove -y && \
-    apt-get clean
-    
-RUN	find /var/lib/apt/lists/ -type f -not -name lock -delete
+    apt-get clean && \
+    find /var/lib/apt/lists/ -type f -not -name lock -delete
 
 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
+
 EXPOSE 30333 9933 9944
 VOLUME ["/substrate"]