add instructions for installing programs in image

readme.md

@@ -18,6 +18,19 @@ a [dockerfile](./Dockerfile) and [compose file](./compose.yaml) have been provid
 keep in mind the commands will run in the docker container, too.
 you do not need to clone this repository to use the compose file provided.
 
+### custom images
+
+to use utilities not installed in the standard docker image,
+you can build a new one based off of it:
+
+```dockerfile
+FROM https://git.alv.cx/alvierahman90/nowblinkie.git
+
+RUN apt-get update && apt-get install blahblahblah
+
+CMD [ "python", "-u", "./src/main.py", "-c", "/config.toml" ]
+```
+
 ## config
 
 the provided [example config file](./config.toml) lists all the options that can be used.