feat(services): Add a service for the lookup server

Signed-off-by: Louis Chmn <louis@chmn.me>

README.md

@@ -85,6 +85,8 @@ Additionally, you can use the following options when starting an instance:
 - `saml`: set up authentik as a SAML IDP.
 - `oidc`: set up authentik as an OIDC IDP.
 - `scim`: set up authentik as a SCIM server.
+- `lookup`: set up a
+  [lookup server](https://github.com/nextcloud/lookup-server).
 - `office`: set up a Nextcloud Office server.
 - `onlyoffice` setup an onlyoffice document server.
 - `push` set up [client push](https://github.com/nextcloud/notify_push).
@@ -293,6 +295,68 @@ name to allow for testing federation between instances. Alternatively, you can
 set up the haze proxy and the proxied domains to get https support between
 instances.
 
+## Global scale
+
+You can start a lookup-server service when starting an instance:
+
+```bash
+haze start --name gs-master lookup
+```
+
+The lookup server is then accessible by other instances at
+`http://haze-gs-master-lookup`.
+
+The Nextcloud instance started with this command is already properly configured
+as master node.
+
+For the slaves instance, here is a sane preset:
+
+```toml
+[[preset]]
+name = "gs-slave"
+apps = ["globalsiteselector"]
+config = {
+	"gs.enabled" = true,
+	"gs.federation" = "global",
+	"gss.jwt.key" = "random-key", # Matches the key set in the docker container of the lookup server.
+	"gss.mode" = "slave",
+}
+```
+
+You'll then have to manually point your slave instances to the lookup and master
+instances:
+
+```bash
+haze start --name gs-slave1 gs-slave
+# Assuming that the master instance was started with `--name gs-master`.
+haze gs-slave1 occ config:system:set gss.master.url --value="http://haze-gs-master.haze.example.com"
+haze gs-slave1 occ config:system:set lookup_server --value="http://haze-gs-master-lookup"
+```
+
+If you want to use the `ManualUserMapping` module, you'll have to set the
+following config on the master instance:
+
+```bash
+haze gs-master occ config:system:set gss.user.discovery.module --value="\\OCA\\GlobalSiteSelector\\UserDiscoveryModules\\ManualUserMapping"
+haze gs-master occ config:system:set gss.discovery.manual.mapping.file --value="/shared/config/gs-user-mapping.json" # See the volume section below for how to mount this file into the container.
+haze gs-master occ config:system:set gss.discovery.manual.mapping.regex --type boolean --value=true
+```
+
+```toml
+[[volume]] # Needed if you are using "ManualUserMapping" module.
+source = "/home/louis/.config/haze/config/gs-user-mapping.json"
+target = "/shared/config/gs-user-mapping.json"
+read_only = true
+```
+
+You can test the connection between the instances and the lookup server by
+running the following command on the slave instance:
+
+```bash
+haze gs-master occ globalsiteselector:discovery
+haze gs-slave1 occ globalsiteselector:discovery
+```
+
 ## Proxy
 
 By default, instances can be accessed by their IP. In order to get more