The base postgis/postgis image does not have PostGIS-related CLIs installed. To use PostGIS CLIs that are NOT installed by default (for example raster2pgsql) it's necessary to extend the base image.
# Create a Docker image
docker build -t my-postgis .
# Run as a Docker container
docker run --name my-postgis -p 5432:5432 -e POSTGRES_USER=admin -e POSTGRES_PASSWORD=password -d my-postgis