Add a nifty script for building statically linked binaries.

We use this script at Kumina to generate binaries that we can deploy to
a wide variety of Linux systems. Let's keep this in the tree, like we do
for some of our other exporters.

build_static.sh

+#!/bin/sh
+
+docker run -i -v `pwd`:/dovecot_exporter alpine:edge /bin/sh << 'EOF'
+set -ex
+
+# Install prerequisites for the build process.
+apk update
+apk add ca-certificates git go libc-dev
+update-ca-certificates
+
+# Build the dovecot_exporter.
+cd /dovecot_exporter
+export GOPATH=/gopath
+go get -d ./...
+go build --ldflags '-extldflags "-static"'
+strip dovecot_exporter
+EOF