dockerfile: Keep track of buildtime metadata

This commit build each images with the following arguments:

* IMAGE: Name of the image (e.g dockcross/base, dockcross/manylinux-x64, ...)
* VCS_REF: dockcross/dockcross commit from which this image is built
* VCS_URL: this repository obtained reading remote.origin.url
* BUILD_DATE: Date and time when the build was initiated

Then, within the Dockerfile, the metadata are associated with the image
using the "LABEL" instruction.
See https://docs.docker.com/engine/reference/builder/#/label

The corresponding labels can be found here:
http://label-schema.org/rc1/#build-time-labels

See #28

browser-asmjs/Dockerfile

@@ -49,3 +49,13 @@ ENV CMAKE_TOOLCHAIN_FILE /usr/emsdk_portable/emscripten/tag-${EMSCRIPTEN_VERSION
 # https://github.com/kripken/emscripten/pull/4477
 RUN sed -i -e '/CMakeForceCompiler/d' -e '/CMAKE_FORCE_C/d' $CMAKE_TOOLCHAIN_FILE
 
+# Build-time metadata as defined at http://label-schema.org
+ARG BUILD_DATE
+ARG IMAGE
+ARG VCS_REF
+ARG VCS_URL
+LABEL org.label-schema.build-date=$BUILD_DATE \
+      org.label-schema.name=$IMAGE \
+      org.label-schema.vcs-ref=$VCS_REF \
+      org.label-schema.vcs-url=$VCS_URL \
+      org.label-schema.schema-version="1.0"