S6: updated to new overlay release 1.9.1.1

https://github.com/just-containers/s6-overlay/releases/tag/v1.19.1.1, which maps to -->
https://github.com/just-containers/skaware/releases/tag/v1.19.1

Most importantly: S6 v2.4.0.0

Alpine: updated to v3.5

• Also added Alpine clean convenience script

CentOS: added as a variant

• Added tests for new variant of CentOS
• Swapped deprecated MAINTAINER tag for LABEL
• Standardized GOSS versions across the board
• Made variant-specific security and clean scripts, symlinked variant's into original place

Scripts: added clean and security_updates conveniences

• Note: only upgrading security packages by default
• Set the terminal mode “properly” (noninteractive)
• Running security update as part of build

Goss: updated to v0.2.4

https://github.com/aelsabbahy/goss/releases/tag/v0.2.4

S6 / Goss:now easily versioned based on env vars

Installed latest release of S6 and Goss

• Fixed bug where cleaning process would wipe out apt signing keys

Goss > Added build-time testing

Merge pull request #6 from bryanlatten/feature-goss

Dockerfile: Added goss

Ubuntu: now using 16.04 (Xenial)

Merge pull request #4 from bryanlatten/feature-removal

Dockerfile: removed error’ing removals on 16.04

Initial Import

Just enough to get S6 process supervision and startup mechanisms in place.