This experimental container-based release of the 'scout' Steam Runtime is enabled on a per-title basis by forcing it's use in the title's Properties dialog.
The Steam Client will understand the following environment variables:
Several environment variables can be used for debug and diagnostics purposes:
STEAM_RUNTIME_CONTAINER_TEST=1 will spawn a 'test mode' dialog allowing you to select a different runtime and set various options.
STEAM_RUNTIME_CONTAINER_ALL=1 will force the container runtime on all titles.
STEAM_RUNTIME_CONTAINER_ALWAYS=1 will enable the container runtime on all titles.
The scripts in pressure-vessel/bin/ support more environment variables to control the behavior.
In particular, setting PRESSURE_VESSEL_WRAP_GUI will enable a test mode interface with more options.