More Docker improvements (#320)

* Docker: install runtime dependencies first

* For better layer caching

* Docker: libpcre3 is already installed

* Docker: use --upgrade and --force-reinstall with pip install

* Docker: copy build files in one go

* Docker: add requirements-dev.txt to dockerignore

.dockerignore

@@ -1,6 +1,7 @@
 .git
 build.bash
 tests
+requirements-dev.txt
 hooks
 scripts
 .*

Dockerfile

@@ -1,10 +1,20 @@
 FROM debian:stretch
 
-COPY requirements.txt /marathon-lb/requirements.txt
-COPY build-haproxy.sh /marathon-lb/build-haproxy.sh
+# runtime dependencies
+RUN apt-get update && apt-get install -y --no-install-recommends \
+        iptables \
+        openssl \
+        procps \
+        python3 \
+        runit \
+        socat \
+    && rm -rf /var/lib/apt/lists/*
+
+COPY requirements.txt build-haproxy.sh \
+    /marathon-lb/
 
 RUN set -x \
-    && buildDeps=" \
+    && buildDeps=' \
         gcc \
         libc6-dev \
         libffi-dev \
@@ -14,22 +24,17 @@ RUN set -x \
         make \
         python3-dev \
         python3-pip \
-        wget \
-    " \
-    && runDeps=" \
-        iptables \
-        libpcre3 \
-        openssl \
-        procps \
-        python3 \
         python3-setuptools \
-        runit \
-        socat \
-    " \
+        wget \
+    ' \
     && apt-get update \
-        && apt-get install -y --no-install-recommends $buildDeps $runDeps \
+        && apt-get install -y --no-install-recommends $buildDeps \
         && rm -rf /var/lib/apt/lists/* \
-    && pip3 install --no-cache -r /marathon-lb/requirements.txt \
+# Install Python packages with --upgrade so we get new packages even if a system
+# package is already installed. Combine with --force-reinstall to ensure we get
+# a local package even if the system package is up-to-date as the system package
+# will probably be uninstalled with the build dependencies.
+    && pip3 install --no-cache --upgrade --force-reinstall -r /marathon-lb/requirements.txt \
     && /marathon-lb/build-haproxy.sh \
     && apt-get purge -y --auto-remove $buildDeps