docker: move the docker image creation inside the main repository

This is a super simple start.
At minimum, configuration via environment variables (see #3543) needs to be
integrated in Etherpad to make this user-friendly.

Resolves #3524.

docker/Dockerfile

@@ -0,0 +1,52 @@
+# Etherpad Lite Dockerfile
+#
+# https://github.com/ether/etherpad-docker
+#
+# Author: muxator
+#
+# Version 0.1
+
+FROM node:latest
+LABEL maintainer="Etherpad team, https://github.com/ether/etherpad-lite"
+
+# install supervisor
+RUN apt-get update && apt-get install -y \
+	supervisor \
+	&& rm -rf /var/lib/apt/lists/*
+
+# install supervisor.conf in a low layer of the container
+ADD supervisor.conf /etc/supervisor/supervisor.conf
+
+# git hash of the version to be built.
+# If not given, build the latest development version.
+ARG ETHERPAD_VERSION=develop
+
+# grab the ETHERPAD_VERSION tarball from github (no need to clone the whole
+# repository)
+RUN echo "Getting version: ${ETHERPAD_VERSION}" && \
+	curl \
+		--location \
+		--fail \
+		--silent \
+		--show-error \
+		--output /opt/etherpad-lite.tar.gz \
+		https://github.com/ether/etherpad-lite/archive/"${ETHERPAD_VERSION}".tar.gz && \
+	mkdir /opt/etherpad-lite && \
+	tar xf /opt/etherpad-lite.tar.gz \
+		--directory /opt/etherpad-lite \
+		--strip-components=1 && \
+	rm /opt/etherpad-lite.tar.gz
+
+# install node dependencies for Etherpad
+RUN /opt/etherpad-lite/bin/installDeps.sh
+
+# Copy the custom configuration file, if present. The configuration file has to
+# be manually put inside the same directory containing the Dockerfile (we cannot
+# directly point to "../settings.json" for Docker's security restrictions).
+#
+# For the conditional COPY trick, see:
+#   https://stackoverflow.com/questions/31528384/conditional-copy-add-in-dockerfile#46801962
+COPY nop setting[s].json /opt/etherpad-lite/
+
+EXPOSE 9001
+CMD ["supervisord", "-c", "/etc/supervisor/supervisor.conf", "-n"]