added documentation

docs/docker.md

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+# Install guide for Docker
+
+```
+docker run -d -p 80:80 -e FOURGET_SERVER_NAME="4get.ca" luuul/4get:latest
+```
+
+...Or with SSL:
+```
+docker run -d -p 443:443 -v /etc/letsencrypt/live/domain.tld:/etc/4get/certs -e FOURGET_SERVER_NAME="4get.ca" luuul/4get:latest
+```
+
+if the certificate files are not mounted to /etc/4get/certs the service listens to port 80
+
+the certificate directory expects files named `fullchain.pem` and `privkey.pem`
+
+# Install using Docker Compose 
+
+copy `docker-compose.yaml`
+
+to serve custom banners create a directory named `banners` for example with images and mount to `/var/www/html/4get/banner`
+
+to serve captcha images create a directory named `captchas` for example containing subfolders with images and mount to `/var/www/html/4get/data/captcha`
+
+any environment variables prefixed with `FOURGET_` will be added to the generated config
+
+the entrypoint will automatically set the `CAPTCHA_DATASET` value for you based on directory names and number of files in each
+
+to set `INSTANCES` pass a comma separated string of urls (FOURGET_INSTANCES = "https://4get.ca,https://domain.tld")
+
+```
+version: "3.7"
+
+services:
+  fourget:
+    image: luuul/4get:latest
+    restart: always
+    environment:
+      - FOURGET_SERVER_NAME=4get.ca
+
+    ports:
+      - "80:80"
+      - "443:443"
+
+    volumes:
+      - /etc/letsencrypt/live/domain.tld:/etc/4get/certs
+      - ./banners:/var/www/html/4get/banner
+      - ./captchas:/var/www/html/4get/data/captcha
+```
+
+Replace relevant values and start with `docker compose up -d`