Documentation for Nix binary cache

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-# Repositories (Docker, Nix...)
+# Repositories (Docker, Nix, Git...)
+
+## Sourcehut
+
+## Gitlab
+
+## Gitea & Gogs
+
+
+## Docker
+
+Not yet compatible, follow [#103](https://git.deuxfleurs.fr/Deuxfleurs/garage/issues/103).
+
+## Nix
+
+Nix has no repository in its terminology: instead, it breaks down this concept in 2 parts: binary cache and channel.
+
+**A channel** is a set of `.nix` definitions that generate definitions for all the software you want to serve.
+
+Because we do not want all our clients to compile all these derivations by themselves,
+we can compile them once and then serve them as part of our **binary cache**.
+
+It is possible to use a **binary cache** without a channel, you only need to serve your nix definitions
+through another support, like a git repository.
+
+As a first step, we will need to create a bucket on Garage and enabling website access on it:
+
+```bash
+garage key new --name nix-key
+garage bucket create nix.example.com
+garage bucket allow nix.example.com --read --write --key nix-key
+garage bucket website nix.example.com --allow
+```
+
+If you need more information about exposing buckets as websites on Garage,
+check [Exposing buckets as websites](/cookbook/exposing_websites.html)
+ and [Configuring a reverse proxy](/cookbook/reverse_proxy.html).
+
+Next, we want to check that our bucket works:
+
+```bash
+echo nix repo > /tmp/index.html
+mc cp /tmp/index.html garage/nix/
+rm /tmp/index.html
+
+curl https://nix.example.com
+# output: nix repo
+```
+
+### Binary cache
+
+To serve binaries as part of your cache, you need to sign them with a key specific to nix.
+You can generate the keypair as follow: 
+
+```bash
+nix-store --generate-binary-cache-key <name> cache-priv-key.pem cache-pub-key.pem
+```
+
+You can then manually sign the packages of your store with the following command:
+
+```bash
+nix sign-paths --all -k cache-priv-key.pem
+```
+
+Setting a key in `nix.conf` will do the signature at build time automatically without additional commands.
+Edit the `nix.conf` of your builder:
+
+```toml
+secret-key-files = /etc/nix/cache-priv-key.pem
+```
+
+Now that your content is signed, you can copy a derivation to your cache.
+For example, if you want to copy a specific derivation of your store:
+
+```bash
+nix copy /nix/store/wadmyilr414n7bimxysbny876i2vlm5r-bash-5.1-p8 --to 's3://nix?endpoint=garage.example.com&region=garage'
+```
+
+*Note that if you have not signed your packages, you can append to the end of your S3 URL `&secret-key=/etc/nix/cache-priv-key.pem`.*
+
+Sometimes you don't want to hardcode this store path in your script.
+Let suppose that you are working on a codebase that you build with `nix-build`, you can then run:
+
+```bash
+nix copy $(nix-build) --to 's3://nix?endpoint=garage.example.com&region=garage'
+```
+
+*This command works because the only thing that `nix-build` outputs on stdout is the paths of the built derivations in your nix store.*
+
+You can include your derivation dependencies:
+
+```bash
+nix copy $(nix-store -qR $(nix-build)) --to 's3://nix?endpoint=garage.example.com&region=garage'
+```
+
+Now, your binary cache stores your derivation and all its dependencies.
+Just inform your users that they must update their `nix.conf` file with the following lines:
+
+```toml
+substituters = https://cache.nixos.org https://nix.example.com
+trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix.example.com:eTGL6kvaQn6cDR/F9lDYUIP9nCVR/kkshYfLDJf1yKs=
+```
+
+*You must re-add cache.nixorg.org because redeclaring these keys override the previous configuration instead of extending it.*
+
+Now, when your clients will run `nix-build` or any command that generates a derivation for which a hash is already present
+on the binary cache, the client will download the result from the cache instead of compiling it, saving lot of time and CPU!
+
+
+### Channels
+
+Channels additionnaly serve Nix definitions, ie. a `.nix` file referencing
+all the derivations you want to serve.