feat: switch to upstream image (#3)

Reviewed-on: https://git.coopcloud.tech/coop-cloud/uptime-kuma/pulls/3
Reviewed-by: yksflip <flip@yksflip.de>
Co-authored-by: p4u1 <p4u1_f4u1@riseup.net>
Co-committed-by: p4u1 <p4u1_f4u1@riseup.net>

README.md

@@ -22,3 +22,10 @@
 * `abra app deploy <app-name>`
 
 For more, see [`docs.coopcloud.tech`](https://docs.coopcloud.tech).
+
+## MariaDB
+
+By default uptime-kuma uses a sqlite database. It is possible to switch to a mariadb:
+```
+#COMPOSE_FILE="$COMPOSE_FILE:compose.mariadb.yml"
+```