Add README

README.adoc

+= Draw.io Docker image
+
+Fedora-based very basic Docker image to run Draw.io in CLI mode, eg. to convert `.drawio` files into `.png` files.
+
+Relies on _xvfb-run_ to create a fake graphical environment, otherwise Draw.io fails to work (even in CLI mode).
+
+NOTE: This image is not built to be lightweight, and is quite heavy. I tried making smaller image using another base than Fedora, but I could not manage to fix problems appearing with fonts when exporting to PNG. That's why I sticked with Fedora.
+
+
+== Using the image
+
+The image is deployed in a link:https://gitlab.univ-nantes.fr/bousse-e/docker-drawio/container_registry[container registry of the University of Nantes Gitlab instance].
+
+To test if the image works:
+
+```
+docker run --rm docker-registry.univ-nantes.fr/bousse-e/docker-drawio drawio --version
+```
+
+To export a `.drawio` file into a `.png`:
+
+```
+docker run --rm -w /data -v $PWD:/data docker-registry.univ-nantes.fr/bousse-e/docker-drawio drawio --export --crop --output out.png in.drawio
+```
+
+Or you can even use the image in a `.gitlab-ci.yml` file to convert your `drawio` files in a CI process − that's actually the reason for this image to exist in the first place :).
+
+
+== Building the image
+
+
+Simply run:
+```
+docker build . -t docker-drawio
+```
+
+Then you can use your locally built image called `docker-drawio` (ie. you can remplace `docker-registry.univ-nantes.fr/bousse-e/docker-drawio` by `docker-drawio` in the previous examples).
\ No newline at end of file