MXE now ships its own version of CMake that sources the toolchain file,
etc. As suggested by their warning when sourcing the toolchain file, use
it.
This also results in updating GCC from 4.9 to 5.4.
As recommended in by CircleCI, explicitly specify the "latest"
tag. This will be important when we will start versioning our images.
Copied from CircleCI documentation [1]:
"Make sure you tag the image you use in the FROM command in your
Dockerfile, even if it is the default “latest” tag. Otherwise, Docker
will pull down all tags for the image you specify. This will create
significant performance problems because the save/load strategy described
above only caches the image layers (and thus tags) that you specify in
the docker save command, so other tags will be re-pulled on every build
if a tag is not specified in the FROM command."
[1] https://circleci.com/docs/docker/#caching-docker-layers
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
