Switch to OpenJDK JRE

Summary:
As Nasqueron Docker engines don't have an Oracle Java SE subscription,
and as the software hosted don't require Java SE specific libraries,
it makes sense to switch to OpenJDK.

Those Docker library community OpenJDK images use Debian as base,
and the current last release for Java 1.8 is 8u242.

Changes from upstream by gizmotronic to Dockerfile are integrated
to transition from Ubuntu to Debian slim image.

Test Plan: Build image

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2173