apps: adapt some documentation for docker-mailserver

apps/draft-real-apps/docker-mailserver/readme.md

@@ -1,4 +1,4 @@
-## mailserver/docker-mailserver
+## Image mailserver/docker-mailserver
 
 A fullstack but simple mail server with SMTP, IMAP, LDAP, Antispam, Antivirus, etc.
 
@@ -20,3 +20,39 @@ To retrieve original Docker image:
 ```
 docker pull mailserver/docker-mailserver:13.2.0
 ```
+
+## Configure & deploy
+
+Most configuration options of the docker-mailserver are available through the Nua deployment configuration. The full list of options is in the [env] tag of the `nua-config.toml` file.
+
+As an example of configuration see the file `sample_deploy.json`:
+
+- The mail is configured for the domain "mail.example.com"
+- With email and password address for the postmaster at "example.com"
+
+
+```json
+{
+    "site": [
+        {
+            "label": "mail",
+            "image": "docker-mailserver",
+            "domain": "mail.example.com",
+            "env": {
+                "POSTMASTER_ADDRESS": "postmaster@example.com",
+                "POSTMASTER_PASSWD": "some_passwd",
+                "OVERRIDE_HOSTNAME": "mail.example.com"
+            }
+        }
+    ]
+}
+```
+
+With this basic config, the mail server will accept mail from the local host on port 25 (without ssl), then send it with DKIM signature.
+
+When deploying the docker-mail server, a `Letsencrypt` certificate is automatically requested, and a DKIM key is generated.
+3 additional configuration files are written in the `/home/nua/letsencrypt` folder to help to the mail configuration on the domain DNS declaration (with "example.com" replaced by the actual domain name):
+
+- `example.com_spf.txt`: a medium strength SPF string (feel free to adapt).
+- `example.com_dmarc.txt`: a medium strength DMARC string (feel free to adapt).
+- `example.com_dkim.pub`: the DKIM public key of this mail server.