What to do if a new version of FoundryVTT is released but the container hasn't been updated yet.

[felddy]

If you cannot wait for a new version of the container to be released you can manually set the FOUNDRY_VERSION environment variable to the version of FoundryVTT you want installed. For example if you want 11.309 you can set the FOUNDRY_VERSION to 11.309 and restart the container. It will log a warning message about the version mismatch, and then happily install the version you requested.

For example:

 ---
 version: "3.8"

 services:
   foundry:
     image: felddy/foundryvtt:release
     hostname: my_foundry_host
     volumes:
       - type: bind
         source: <your_data_dir>
         target: /data
     environment:
       - FOUNDRY_ADMIN_KEY=atropos
       - FOUNDRY_PASSWORD=N0wN0wNOW
       - FOUNDRY_USERNAME=TheBleedingEdge
       - FOUNDRY_VERSION=11.309
     ports:
       - target: 30000
         published: 30000
         protocol: tcp

[Mitchell-Faas]

Could you elaborate on the downsides of doing so? Would you risk compatibility issues / corruption, for example?

(If there are no risks I'm wondering why this isn't the default way to configure the image)

[felddy]

This is indeed a higher risk path. Since the container hasn't been tested with exact release you are setting, there could be compatibility issues. Since the container only manages the install of Foundry and the creation on configuration files, I wouldn't expect that it would cause corruption. That said, it is always best to backup your data volume before making any experimental changes.