Allow library/plugin injection via volumes

Why:

• Customers often need to incorporate additional library and atlassian-plugin JAR files when they run the products (whether via Docker or otherwise). A common example is to add JDBC driver libraries that cannot be shipped with the products for license reasons.
• This is currently difficult to do with our Docker images. You need to start the container with a persistent volume mount, copy the new JAR file(s) into the volume, then restart the container.
• It's also awkward to handle with Kubernetes, requiring the fiddling use of subPath volume mounts.

What:

• Add support for new environment variables in the Docker images which specify a volume mount location in the container which contains additional lib/plugin JAR files.
• During first container startup, copy the contents of these mounts into the appropriate place for library or plugin JARs.