make Docker image smaller + use debian:buster-slim base

This commit updates the Dockerfile to use multiple stages. In the first stage, compile-time dependencies are installed, and libzmq is compiled and tested. The second stage starts with a fresh, slim Docker image, and the compiled outputs of the first stage are copied over. Runtime dependencies, like libsodium and kerberos, are installed as well.

Dockerfile

@@ -1,11 +1,32 @@
-FROM ubuntu:14.04
+FROM debian:buster-slim AS builder
+LABEL maintainer="ZeroMQ Project <zeromq@imatix.com>"
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update -qq \
+    && apt-get install -qq --yes --no-install-recommends \
+        autoconf \
+        automake \
+        build-essential \
+        git \
+        libkrb5-dev \
+        libsodium-dev \
+        libtool \
+        pkg-config \
+    && rm -rf /var/lib/apt/lists/*
+WORKDIR /opt/libzmq
+COPY . .
+RUN ./autogen.sh \
+    && ./configure --prefix=/usr/local --with-libsodium --with-libgssapi_krb5 \
+    && make \
+    && make check \
+    && make install
 
-MAINTAINER ZeroMQ Project <zeromq@imatix.com>
-
-RUN apt-get update
-RUN DEBIAN_FRONTEND=noninteractive apt-get install -y git build-essential libtool autoconf automake pkg-config unzip libkrb5-dev
-RUN cd /tmp && git clone git://github.com/jedisct1/libsodium.git && cd libsodium && git checkout e2a30a && ./autogen.sh && ./configure && make check && make install && ldconfig
-RUN cd /tmp && git clone --depth 1 git://github.com/zeromq/libzmq.git && cd libzmq && ./autogen.sh && ./configure && make
-# RUN cd /tmp/libzmq && make check
-RUN cd /tmp/libzmq && make install && ldconfig
-RUN rm /tmp/* -rf
+FROM debian:buster-slim
+LABEL maintainer="ZeroMQ Project <zeromq@imatix.com>"
+ARG DEBIAN_FRONTEND=noninteractive
+RUN apt-get update -qq \
+    && apt-get install -qq --yes --no-install-recommends \
+        libkrb5-dev \
+        libsodium23 \
+    && rm -rf /var/lib/apt/lists/*
+COPY --from=builder /usr/local /usr/local
+RUN ldconfig && ldconfig -p | grep libzmq