[skip ci] Add documentation

docker/CONTRIBUTING.md

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+# Contributing
+
+## CMake installation in Dockerfiles
+
+As the project requires CMake v3.23 at least, and as some Dockerfiles are based on Ubuntu 20.04 images, CMake has to be installed manually.
+The installer is downloaded, its signature is verified, then its checksum is verified.
+To check the signature, the public key of the person who signed the binary is required.
+This public key can be extracted from the key ID.
+
+When updating the Dockerfiles for a newer version of CMake (if needed), the process to get the right public key is as follows:
+
+1. Identify the release on GitHub;
+2. Copy the key ID in the line "PGP sign by XXXXXXXX";
+3. Paste it in `https://keys.openpgp.org/` to retrieve the URL of the public key file;
+4. Copy the last part in the URL `https://keys.openpgp.org/vks/v1/by-fingerprint/YYYYYYYY`;
+5. Update the Dockrfiles with this value.

docker/README.md

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+# Dockerfiles
+
+Those Dockerfiles are mainly used for CI.
+Each backend/compiler has a corresponding Dockerfile.