docker: pin microsoft/mssql-server-linux to 2017-CU4 tag

docker/blocks/mssql/build/Dockerfile

@@ -1,4 +1,4 @@
-FROM microsoft/mssql-server-linux:latest
+FROM microsoft/mssql-server-linux:2017-CU4
 WORKDIR /usr/setup
 COPY . /usr/setup
 RUN chmod +x /usr/setup/setup.sh