docker: Add postgres docker build with full text search.

This is multi-stage build which first builds tsearch-extras with the
current version of postgres and then configs postgres for zulip. The
zulip config installs the hunspell dictionaries, stop words file,
tsearch-extras, and creates the initial database.

**Testing Plan:**
1) `docker-compose up` the existing config.
2) Build the new image
3) Edit docker-compose.yml to use the new image id
4) `docker-compose up` and verify full text search is still working.

scripts/setup/postgres-create-db

@@ -0,0 +1,21 @@
+#!/bin/bash
+set -e
+set -x
+
+# Make sure the current working directory is readable
+cd /
+
+DATABASE_CREATE="
+CREATE USER zulip;
+ALTER ROLE zulip SET search_path TO zulip,public;
+CREATE DATABASE zulip OWNER=zulip;
+\\connect zulip
+CREATE SCHEMA zulip AUTHORIZATION zulip;
+CREATE EXTENSION tsearch_extras SCHEMA zulip;
+"
+
+if [ -f /.dockerenv ]; then
+    echo "$DATABASE_CREATE" | psql
+else
+    echo "$DATABASE_CREATE" | su postgres -c psql
+fi