8.5.0: Nginx: using cont-finish.d for more graceful shutdown (#67)

Changed:

• Inline startup check for nginx syntax. Prevents bad env variable inputs from producing invalid config

Shutdown behavior:

• Used according to newly-clarified + documented shutdown behavior: https://github.com/behance/docker-base#shutdown-behavior, exchanged implementation to /etc/cont-finish.d
• Set drain timeout to 55s, deliberately <60s to account for lack of upstream/downstream timing precision