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+## Overall Docker Setup for Coq's CI.
+
+This directory provides Docker images to be used by Coq's CI. The
+images do support Docker autobuild on `hub.docker.com` and Gitlab's
+private registry.
+
+Gitlab CI will build and tag a Docker by default for every job if the
+`SKIP_DOCKER` variable is not set to `false`. In Coq's CI, this
+variable is usually set to `false` indeed to avoid booting a useless
+job.
+
+## Manual Building
+
+You can also manually build and push any image:
+
+- Build the image `docker build -t base:$VERSION .`
+
+To upload/push to your hub:
+
+- Create a https://hub.docker.com account.
+- Login into your space `docker login --username=$USER`
+- Push the image:
+  + `docker tag base:$VERSION $USER/base:$VERSION`
+  + `docker push $USER/base:$VERSION`
+
+## Debugging / Misc
+
+To open a shell inside an image do `docker run -ti --entrypoint /bin/bash <imageID>`
+
+Each `RUN` command creates an "layer", thus a Docker build is
+incremental and it always help to put things updated more often at the
+end.
+
+## Possible Improvements:
+
+- Use ARG for customizing versions, centralize variable setup;