add guide for issuing Let's Encrypt certificate with Docker

docs/issuing-letsencrypt-certificate.md

@@ -0,0 +1,53 @@
+## Issuing a Let's Encrypt certificate
+
+**NOTE:** Commands with a **$** prefix denote those are executed as user, **#** as root and commands without a prefix are database commands.
+
+For issuing a Let's Encrypt certificate one can use Docker as well which will save you from messing around with
+installing on the host system.
+This guide assumes you're inside the mattermost-docker directory but if using absolute paths in the volume bind mounts
+(e.g. /home/admin/mattermost-docker instead of `${PWD}`) it doesn't matter because the paths are unique. These commands
+requires that DNS records (A or CNAME) have been set and resolve to your server's external IP.
+
+1. Issuing the certificate using the standalone authenticator (because there is no nginx yet)
+```
+$ sudo docker run -it --rm --name certbot -p 80:80 \
+    -v "${PWD}/certs/etc/letsencrypt:/etc/letsencrypt" \
+    -v "${PWD}/certs/lib/letsencrypt:/var/lib/letsencrypt" \
+    certbot/certbot certonly --standalone -d mm.example.com
+```
+
+2. Changing the authenticator to webroot for later renewals
+
+```
+$ sudo docker run -it --rm --name certbot \
+    -v "${PWD}/certs/etc/letsencrypt:/etc/letsencrypt" \
+    -v "${PWD}/certs/lib/letsencrypt:/var/lib/letsencrypt" \
+    -v shared-webroot:/usr/share/nginx/html \
+    certbot/certbot certonly -a webroot -w /usr/share/nginx/html -d mm.example.com
+```
+
+This will ask you to abort or renew the certificate. When choosing to renew `certbot` will alter the renewal
+configuration to *webroot*.
+As an alternative (which will save you one certificate creation request https://letsencrypt.org/docs/rate-limits/) this can be done by yourself with the following commands
+
+```
+$ sudo sed -i 's/standalone/webroot/' ${PWD}/certs/etc/letsencrypt/renewal/mm.example.com.conf
+$ sudo tee -a ${PWD}/certs/etc/letsencrypt/renewal/mm.example.com.conf > /dev/null << EOF
+webroot_path = /usr/share/nginx/html,
+[[webroot_map]]
+EOF
+```
+
+3. Command for requesting renewal (Let's Encrypt certificates do have a 3 month lifetime)
+
+```
+sudo docker run --rm --name certbot \
+    --network mattermost \
+    -v "${PWD}/certs/etc/letsencrypt:/etc/letsencrypt" \
+    -v "${PWD}/certs/lib/letsencrypt:/var/lib/letsencrypt" \
+    -v shared-webroot:/usr/share/nginx/html \
+    certbot/certbot renew --webroot-path /usr/share/nginx/html
+```
+
+This command can be called with a systemd timer on a regulary basis (e.g. once a day). Please take a look at the
+*contrib/systemd* folder.