feat(Dockerfile): Use multistage build to bring in pbf executable

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
pelias/docker-baseimage#23 as well, will be only
322MB. That's a nice 600MB savings!

Before pelias/docker-baseimage#23 the image size
still drops to 500MB, still a healthy reduction.

Replaces #262

Dockerfile

@@ -1,20 +1,27 @@
-# base image
-FROM pelias/baseimage
+# use multi-stage build to make pbf binary
+FROM pelias/baseimage as builder
 
-# change working dir
-ENV WORKDIR /code/pelias/polylines
-WORKDIR ${WORKDIR}
+RUN apt-get update && apt-get install gcc -y
 
 # install Golang
-ENV GOPATH=/usr/src/.go
+ENV GOPATH=/go
 RUN wget -qO- https://golang.org/dl/go1.15.2.linux-amd64.tar.gz | tar -C /usr/local -xzf -
-RUN mkdir -p "${GOPATH}"
-ENV PATH="${PATH}:/usr/local/go/bin:${GOPATH}/bin"
+ENV PATH="${PATH}:/usr/local/go/bin"
 ENV GO111MODULE=on
 
-# get go dependencies
+# install pbf dependency
 RUN go get github.com/missinglink/pbf
 
+# use Pelias baseimage for the main image
+FROM pelias/baseimage
+
+# change working dir
+ENV WORKDIR /code/pelias/polylines
+WORKDIR ${WORKDIR}
+
+# copy pbf binary from builder container
+COPY --from=builder /go/bin/pbf /bin/
+
 # copy package.json first to prevent npm install being rerun when only code changes
 COPY ./package.json ${WORKDIR}
 RUN npm install