Say you want to support large images or different image formats. These are defined in configuration files listed here. You can override the defaults set by Platform.sh by providing one or more of these files and placing them in a custom path stored in MAGICK_CONFIGURE_PATH variable.
For example, if you have a custom policy.xml file defining how to process large files:
Place the policy.xml in a location which is not publicly accessible (for example, in /app/imagemagick directory).
Create an environment variable env:MAGICK_CONFIGURE_PATH with the value of /app/your_policy_directory as the location. In .platform.app.yaml:
variables: env: MAGICK_CONFIGURE_PATH: "/app/imagemagick"
Afterwards, your custom settings should override the defaults.