Add a README for creating CI containers

Author: ethomson

README.md

@@ -0,0 +1,29 @@
+Docker images for libgit2 continuous integration builds
+=======================================================
+[libgit2](https://libgit2.org/)'s Linux-based pull request validation,
+post-merge continuous integration and nightly builds run inside Docker
+containers.  This ensures that we can target a predictable platform
+running exactly the distribution with exactly the dependencies that
+we want.
+
+We target amd64-based containers natively, and other platforms like
+x86 and arm by leveraging QEMU.
+
+To build and test these locally, you can simply run `make`.  This will
+produce a new set of docker images with the `test` tag.  You can then
+run them locally.  From within a [libgit2 source
+directory](https://github.com/libgit2/libgit2), you can test the CI
+build step:
+
+    docker run -v $(pwd):/src -v $(pwd)/build:/build -w /build libgit2/trusty-amd64:test /src/ci/build.sh
+
+You can also run the unit tests:
+
+    docker run -v $(pwd):/src -v $(pwd)/build:/build -w /build libgit2/trusty-amd64:test /src/ci/test.sh
+
+(Replace `libgit2/trusty-amd64:test` with the name of the image that you
+want to run.)
+
+When you've validated the images, you can build release images and push
+them to dockerhub.  To build release images, you can run `make TAG=latest`.
+To push them, you can run `make TAG=latest push`.