Multistage build

[flavioaiello]

With multi-stage builds, multiple FROM statements can be used to separate source- and runtime-build. The resulting docker image is very small and does not contain obsolete libs, thus reducing the potential attack surface.
Haproxy as a very mature and leading docker image was setup many years ago. This pull request should initiate the switch to multistage build.

[tianon]

I'm definitely not seeing size savings from this; the current build of haproxy:1.8-alpine is ~16.8MB, and this build clocks in at ~21MB.

See also docker-library/official-images#3383.

[tianon]

I've gotten it a little better by removing the Alpine haproxy package and not doing apk update or apk upgrade, but it's still ~17.7MB and doesn't run properly (because it's missing at least libpcre.so.1, possibly other libraries too).

[flavioaiello]

I tought libpcre is only needed during build- and not on runtime. I pushed an update removing the haproxy apk package and the update/upgrade step. Imho using multistage-build increases usability.

[yosifkit]

We cannot use multi-stage builds at this time: docker-library/official-images#3383.