Use multistage build to bring in `pbf` executable

[orangejulius]

The polylines Docker image is a bit of a large one currently, as it includes not just Node.js and a node_modules directory, but a full compiler toolchain, an install of the Go language, the dependencies of the pbf repository from https://github.com/missinglink/pbf, and the final pbf executable that comes from it.

All told, this brought the total image size to a whopping 950MB uncompressed.

This PR makes use of multi stage builds to run the compiling of the pbf executable in a separate container. After this, all the toolchain and dependencies needed can be thrown away, and only the small executable copied to the final image.

Using container-diff it looks like the image size, uncompressed, after https://github.com/pelias/docker-baseimage/pull/23 as well, will be only 322MB. That's a nice 600MB savings!

Before https://github.com/pelias/docker-baseimage/pull/23 the image size still drops to 500MB, also a healthy reduction.

Replaces https://github.com/pelias/polylines/pull/262

[missinglink]

Yeah very nice

[orangejulius]

Some quick notes:

• this works great with pelias prepare polylines
• I chose pelias/baseimage as the builder image for general consistency, and specifically to use a glibc-based environment. Alpine uses musl and incompatibilities between it and glibc can be real nasty.