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

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+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" ]

scripts/docker/README.md

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+# smp-server docker container
+0. Install `docker` to your host.
+
+1. Build your `smp-server` image:
+```sh
+DOCKER_BUILDKIT=1 docker build -t smp-server .
+```
+
+2. Run new docker container:
+```sh
+docker run -d \
+	--name smp-server \
+	-e addr="your_ip_or_domain" \
+	-p 5223:5223 \
+	-v ${PWD}/config:/etc/opt/simplex \
+	-v ${PWD}/logs:/var/opt/simplex \
+	smp-server
+```
+
+Configuration files and logs will be written to [`config`](./config) and [`logs`](./logs) folders respectively.

scripts/docker/config/README.md

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+smp-server configuration, certificate and fingerprint will be stored here

scripts/docker/entrypoint

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+#!/usr/bin/env sh
+confd="/etc/opt/simplex"
+logd="/var/opt/simplex/"
+
+# Check if server has been initialized
+if [ ! -f "$confd/smp-server.ini" ]; then
+	# If not, determine ip or domain
+	case $addr in
+		'') printf "Please specify \$addr environment variable.\n"; exit 1 ;;
+		*[a-zA-Z]*) smp-server init -l -n "$addr" ;;
+		*) smp-server init -l --ip "$addr" ;;
+	esac
+
+fi
+
+# backup store log
+[ -f "$logd/smp-server-store.log" ] && cp "$logd"/smp-server-store.log "$logd"/smp-server-store.log.bak
+# rotate server log
+[ -f "$logd/smp-server.log" ] && mv "$logd"/smp-server.log "$logd"/smp-server-"$(date +'%FT%T')".log
+
+# Finally, run smp-sever. Notice that "exec" here is important:
+# smp-server replaces our helper script, so that it can catch INT signal
+exec smp-server start > "$logd"/smp-server.log 2>&1

scripts/docker/logs/README.md

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+smp-server general logs, stored messages and statistics (if enabled) will be stored here