chore: improvements to keycloak local development (#26518)

* update keycloak docker image

* update realm file with a compatible realm

* import realm on start-docker command

Since bitnami's image does not support importing directly, the import of the test realm is done in the make file start-docker action

* Use official image from quay

* updated realm keycloak config

* final note about nickname attrib for saml

* add admin user

* update realm

* Updated from master

* Updated docs

* local typo

* use jq for ldap and saml

* updated readme

server/build/docker/keycloak/openid.mmsettings.json

@@ -0,0 +1,14 @@
+{
+    "OpenIdSettings": {
+        "Enable": true,
+        "Secret": "9Y7dykcoA9luTC77XtXxOu9UbNx3rhj6",
+        "Id": "mattermost-openid",
+        "Scope": "profile openid email",
+        "AuthEndpoint": "",
+        "TokenEndpoint": "",
+        "UserAPIEndpoint": "",
+        "DiscoveryEndpoint": "http://localhost:8484/realms/mattermost/.well-known/openid-configuration",
+        "ButtonText": "Login using OpenID",
+        "ButtonColor": "#ffaa4c"
+    }
+}