Move baseimage from Node.js 12 to mainline (Node.js 12)

The libpostal baseimage has historically stayed on the Node.js 10 branch
of the Pelias base image due to incompatibility between [node-postal and
Node.js 12](openvenues/node-postal#24).

We are now using a Node.js 12 compatible fork of the `node-postal`
libary, so we can now bring the libpostal baseimage back to parity with
the other Docker images.

This will reduce complexity, maintenance times, and download sizes for
Pelias containers.

Connects pelias/interpolation#245
Connects pelias/pelias#800

Dockerfile

@@ -1,5 +1,5 @@
 # base image
-FROM pelias/baseimage:nodejs-10
+FROM pelias/baseimage
 
 # libpostal apt dependencies
 # note: this is done in one command in order to keep down the size of intermediate containers