docs: compile a list of external dependencies and their versions across the environments

Fix #1585

Author: php-coder

docs/external-dependencies.md

@@ -0,0 +1,11 @@
+# External Dependencies
+
+| Component  | Production               | Travis CI             | docker-compose                           |
+| -------    | ------------------------ | --------------------- | ---------------------------------------- |
+| OS         | Ubuntu 16.04.07 (Xenial) | Ubuntu 16.04 (Xenial) |                                          |
+| JDK        | OracleJDK 8u151          | OpenJDK 8u292         | OpenJDK 8u121-jre on Debian 8.11 (Jessy) |
+| MySQL      | 5.7.27                   | 5.7.33                | 5.7.20 on Debian 8.11 (Jessy)            |
+| PostgreSQL |                          | 10.17                 | 11.3 on Alpine 3.9                       |
+| Python     | 2.7.12, 3.5.2            | 2.7.12, 3.5.2, 3.6.10 |                                          |
+| bash       | 4.3.14                   | 4.3.48                |                                          |
+| duplicity  | 0.7.06                   |                       |                                          |