add hosted docker container (#479)

* add docker container

* Apply suggestions from code review

* Update README.md

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

scripts/docker/Dockerfile

@@ -0,0 +1,20 @@
+FROM ubuntu:focal
+
+# Install curl 
+RUN apt-get update && apt-get install -y curl
+
+# Download latest smp-server release and assign executable permission
+RUN curl -L https://github.com/simplex-chat/simplexmq/releases/latest/download/smp-server-ubuntu-20_04-x86-64 -o /usr/bin/smp-server && \
+	chmod +x /usr/bin/smp-server
+
+# Copy our helper script
+COPY ./entrypoint /usr/bin/entrypoint
+
+# Open smp-server listening port
+EXPOSE 5223
+
+# SimpleX requires using SIGINT to correctly preserve undelivered messages and restore them on restart
+STOPSIGNAL SIGINT
+
+# Finally, execute helper script
+ENTRYPOINT [ "/usr/bin/entrypoint" ]