docker: add README file

docker/README.md

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+# Container images for Netdev tests
+
+The idea here is to have some container images to get an environment closed to
+the CI one, to easily run the same tests as the ones executed on NIPA.
+
+Instead of installing a bunch of (undocumented) dependences, and trying to guess
+which versions are used on NIPA, these containers can be used.
+
+These images are supposed to be easy to use and maintain. They are automatically
+built and pushed to the GitHub registry each time the related files are
+modified: <https://github.com/orgs/linux-netdev/packages>.
+
+The Dockerfile are also kept simple to let more people modifying them if needed:
+at the end it is just a bunch of commands someone would execute to install
+programs on their side on Fedora.
+
+## Usage
+
+To use them, that's easy, either:
+
+- Execute `docker/build/run.sh` (static tests) or `docker/selftests/run.sh`
+  (runtime tests) with `--pull`, then execute the same commands as before, but
+  from the containers.
+
+- Use the same scripts followed by the same commands as before to execute them
+  directly from the containers.
+
+- Use Docker or Podman directly, e.g.
+
+  ```console
+  $ mkdir -p .ccache
+  $ docker run \
+    -v .ccache:/home/nipa/.ccache:rw \
+    -v "${PWD}:${PWD}:rw" \
+    -w "${PWD}" \
+    -u "${RUID:-$(id -u)}:${RGID:-$(id -g)}" \
+    --group-add "$(grep "^kvm:" /etc/group | cut -d: -f3)" \
+    --rm \
+    -i -t \
+    --privileged \
+    --pull always \
+    ghcr.io/linux-netdev/nipa-selftests:latest \
+    ## the rest of the command here
+  ```
+
+For more details about which commands to execute next, please check the wiki:
+[Running Netdev CI tests locally](https://github.com/linux-netdev/nipa/wiki/Running-Netdev-CI-tests-locally).
+
+## Images
+
+In this repository, there are different images:
+
+- `base`: a common base for the following ones, on top of Fedora, latest version
+- `build`: containing dependencies for the static tests, to run `ingest_mdir.py`
+- `selftests`: containing dependencies for the runtime tests: KSelftests, KUnit
+
+Note: `debian` is a deprecated image to run the static tests. The Netdev CI is
+using a Fedora base, so it is recommended to do the same.
+
+## Build locally
+
+If you prefer to build container images locally, you can use the
+`docker/build.sh` script.