feat(#388): Updated docker image. Closes #388

Dockerfile

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+FROM ruby:2.3.5
+MAINTAINER Serpico
+
+ENV SRP_ROOT /Serpico
+WORKDIR $SRP_ROOT
+COPY . $SRP_ROOT
+
+RUN bundle install
+
+# Allow DB to be on a shared volume
+VOLUME ["$SRP_ROOT/db"]
+EXPOSE 8443
+
+CMD ["bash", "scripts/docker.sh"]

docs/docker.md

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+# Running Inside Docker
+
+The included `Dockerfile` allows to run Serpico inside docker from any system
+that supports containers.
+
+By default, Serpico listens on 8443, you can expose it as `443` if you would
+like by using `docker run -p 443:8443 ...`
+
+The image needs to first be built.
+
+1. Build the image
+2. Map the database location in docker-compose or at `docker run` time.
+3. If the database doesn't exist, it will be created with defaults
+
+## Creating the image
+
+This will create a container with the current state of your repository.
+The database is not created at this point, and this image is safe for
+distribution.
+
+```
+docker build -t serpico .
+```
+
+The Dockerfile exposes a `VOLUME` at `/Serpico/db` to allow mounting an
+external database through `docker-compose` or `docker run -v`.
+
+
+## Running with `docker run`
+
+```
+# In the foreground
+docker run --rm -p 8443 -v"$(pwd)":/Serpico/db -it serpico
+```
+
+This will store the database locally at `$PWD/master.db` Please note that the
+path to the database on the host [must be absolute][1].
+
+[1]: https://docs.docker.com/engine/reference/run/#volume-shared-filesystems
+
+## Caveats
+
+This is a work in progress, so a few things are currently not supported.
+
+- Running a new container with an existing `master.db` will not work because
+  `first_time.rb` will not run, and there won't be any certificates for SSL.
+- `config.json` is not exposed to the host so customization requires rebuilding
+  the image or accessing it with `docker exec bash`.
+

scripts/docker.sh

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+#!/usr/bin/bash
+# This script is used as the entry point for the docker image.
+# It will initialize the database if it isn't already present.
+
+if [ ! -f "$SRP_ROOT/db/master.db" ]; then
+    echo "First run detected. Initializing database..."
+    ruby "$SRP_ROOT/scripts/first_time.rb"
+fi
+
+# CMD ["ruby", "serpico.rb"]
+ruby serpico.rb
+