Added dockerfile for osx build

(for safe keeping)

gitlab-ci/osxcross_docker/Dockerfile

+FROM debian:stretch
+MAINTAINER Matteo De Carlo <matteo.dek@covolunablu.org>
+
+# Install build tools
+RUN apt-get update && \
+    DEBIAN_FRONTEND=noninteractive apt-get upgrade -yy && \
+    DEBIAN_FRONTEND=noninteractive apt-get install -yy \
+        automake            \
+        bison               \
+        curl                \
+        file                \
+        flex                \
+        git                 \
+        libtool             \
+        pkg-config          \
+        python              \
+        texinfo             \
+        vim                 \
+        gcc                 \
+        g++                 \
+        cmake               \
+        wget
+
+# Install osxcross
+ENV OSXCROSS_SDK_VERSION 10.11
+ENV MAKEFLAGS -j64
+RUN mkdir /opt/osxcross &&                                      \
+    cd /opt &&                                                  \
+    git clone https://github.com/tpoechtrager/osxcross.git &&   \
+    cd osxcross &&                                              \
+    git checkout 474f359d2f27ff68916a064f0138c9188c63db7d &&    \
+    ./tools/get_dependencies.sh &&                              \
+    yes | INSTALLPREFIX=/usr ./build_clang.sh &&                \
+    cd /opt/osxcross/build/llvm-3.9.1.src/build_stage2 && make install && \
+    cd - && rm -rf /opt/osxcross/build && \
+    curl -L -o ./tarballs/MacOSX${OSXCROSS_SDK_VERSION}.sdk.tar.xz \
+         https://s3.amazonaws.com/andrew-osx-sdks/MacOSX${OSXCROSS_SDK_VERSION}.sdk.tar.xz && \
+    yes | PORTABLE=true ./build.sh &&                           \
+    ./build_compiler_rt.sh
+
+## alternative SDK sources:
+# https://s3.amazonaws.com/andrew-osx-sdks/MacOSX${OSXCROSS_SDK_VERSION}.sdk.tar.xz
+# https://github.com/phracker/MacOSX-SDKs/releases/download/10.13/MacOSX${OSXCROSS_SDK_VERSION}.sdk.tar.xz
+
+ENV PATH $PATH:/opt/osxcross/target/bin
+CMD /bin/bash