mailcow-dockerized-docs/docs/u_e-docker-cust_dockerfiles.md

You need to copy the override file with corresponding build tags to the mailcow: dockerized root folder (i.e. /opt/mailcow-dockerized):

cp helper-scripts/docker-compose.override.yml.d/BUILD_FLAGS/docker-compose.override.yml docker-compose.override.yml

Make your changes in data/Dockerfiles/$service and build the image locally:

docker build data/Dockerfiles/service -t mailcow/$service

Now auto-recreate modified containers:

docker-compose up -d