update README and add docker file to this repo

README.md

@@ -3,15 +3,8 @@
 This repository contains the shared Iris CI. It contains common code for Iris
 projects' CI.
 
-Projects automatically use the latest commit of *some branch* for their CI, so
-branches should remain reasonably compatible.  Every branch here comes with a
-corresponding branch [in the Docker repo](https://github.com/RalfJung/opam-ci),
-providing the Docker image that should be used.  Currently, we have the
-following branches:
-
-* [`opam2`](https://gitlab.mpi-sws.org/FP/iris-ci/tree/opam2), using opam 2.0.
-
-Always remember to push to the right branch!
+Projects automatically use the latest commit for their CI, so every change needs
+to maintain compatibility and breaking changes need to have a migration plan.
 
 ## How to configure CI for your project
 

docker/Dockerfile

+# Keep distro in sync with CI builder! We run perf inside the container, needs to work with host kernel.
+FROM debian:buster
+
+# apt package installation
+# gmp: needed by Coq
+# python: needed by z3 (for RefinedC)
+# mpfr: needed by Cerberus (for RefinedC)
+# pkg-config: needed by the conf-mpfr opam package (related to the use of mpfr by Cerberus)
+RUN apt update -y && apt install -y pkg-config git rsync tar unzip m4 time curl linux-perf strace ocaml build-essential bubblewrap gawk libgmp-dev python2.7 python3 libmpfr-dev
+
+# opam installation
+RUN curl "https://github.com/ocaml/opam/releases/download/2.1.3/opam-2.1.3-x86_64-linux" -Lo /usr/local/bin/opam && chmod +x /usr/local/bin/opam
+
+# prepare CI environment
+WORKDIR /
+RUN useradd -m ci
+USER ci

docker/README.md

+This is the docker image for Iris CI.
+
+Here's how to update this image on Docker Hub:
+```
+docker build -t ralfjung/opam-ci:opam2 .
+docker login
+docker push ralfjung/opam-ci:opam2
+```