Add a dockerfile for CI to run in

docker/Dockerfile

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+FROM debian:buster-slim
+
+RUN set -ex \
+    && apt-get update \
+    && apt-get install -y \
+	build-essential \
+	python2.7-dev \
+	libffi-dev \
+	sqlite3 \
+	python-pip \
+	libssl-dev \
+	python-virtualenv \
+	libxslt1-dev
+
+# Install dependencies
+RUN pip install https://github.com/matrix-org/sydent/tarball/master \
+    && pip install --upgrade pip \
+    && pip install --upgrade setuptools
+
+# Copy in and run the start script
+COPY ./sydent-ci.sh /sydent-ci.sh

docker/README.md

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+# Docker
+
+Currently only a single, official docker container
+(`matrixdotorg/sydent-build`) is provided. It is intended to set up a running
+environment for testing Sydent and nothing more. You can find it [on Docker
+hub](https://hub.docker.com/r/matrixdotorg/sydent-build).

docker/sydent-ci.sh

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+#!/usr/bin/env bash
+
+# Install any new dependencies
+pip install -e .
+
+# Start Sydent and run for 5s
+timeout 5s python -m sydent.sydent
+
+# Check that it started up correctly
+if [ $? -eq 124 ]; then
+  # Timeout got it, so it must've been running correctly
+  echo "Sydent startup up successfully"
+  exit 0
+else
+  # Sydent exited before the timeout. Something went wrong
+  echo "Sydent failed to start"
+  echo 1
+fi