There are a bunch of ccache environment variables that the Flatpak SDK should keep. It seems that CCACHE_ prefix should be added to the ones already being considered.
Created attachment 400036 [details]
Created attachment 400037 [details]
Committed r262052: <https://trac.webkit.org/changeset/262052>
All reviewed patches have been landed. Closing bug and clearing flags on attachment 400037 [details].