Makefile: build promtail with CGO_ENABLED if GOHOSTOS=GOOS=linux (#809)

This commit modifies the Makefile to build promtail with CGO_ENABLED if
both GOHOSTOS and GOOS are set to linux.

To facilitate the Docker container utilizing the journal support,
the container for promtail had to be modified to be based on top of
debian. The sd-journal library depends on libsystemd.so being present
at runtime. As alpine currently does not have a package for systemd,
the easiest approach is to change the base image to debian:stretch-slim.
A future solution for providing an alpine-based build will be considered
in the future and discussion for the approach will be tracked in #838.

This commit also updates the Dockerfile to default to using the
latest loki-build-image. If a specific version is required, it should
be provided as a build arg to docker build.