Certain options which are valid for “build-webkit” are never passed down
to the instance that runs inside the Flatpak sandbox. For example, happens
with “--cmakeargs=...” (output reformatted, to ease reading):

  % build-webkit --gtk --release --makeargs='-j32'                          
  Building flatpak based environment
  Building webkit
  Running in sandbox: "flatpak" "build" "--die-with-parent" "--bind-mount=/run/shm=/dev/shm"
     "--bind-mount=/etc/perl=/home/aperez/WebKit/WebKitBuild/GTK/FlatpakTreeRelease/files/lib/perl"
     "--bind-mount=/run/host//tmp=/tmp" "--bind-mount=/app/webkit=/home/aperez/WebKit"
     "--bind-mount=/app/webkit/WebKitBuild/Release=/home/aperez/WebKit/WebKitBuild/GTK/Release"
     "--env=LANG=en_US.utf8" "--env=WEBKIT_TOP_LEVEL=/app/"
     "--env=TEST_RUNNER_INJECTED_BUNDLE_FILENAME=/app/webkit/lib/libTestRunnerInjectedBundle.so"
     "--share=ipc" "--socket=x11" "--socket=wayland" "--device=all" "--share=network" "--socket=pulseaudio"
     "--system-talk-name=org.freedesktop.GeoClue2" "--filesystem=host" "--socket=system-bus"
     "--talk-name=org.freedesktop.Flatpak" "--env=GST_PRESET_PATH=/app/share/gstreamer-1.0/presets/" 
     "/home/aperez/WebKit/WebKitBuild/GTK/FlatpakTreeRelease" "/app/webkit/Tools/Scripts/build-webkit"
     "--release" "--gtk" "--prefix=/app"

  +  cmake --build /app/webkit/WebKitBuild/Release --config Release -- 
  ...

As per Thibault's suggestion, I tried “webkit-flatpak” insted, to no avail:

  % webkit-flatpak --release --gtk --command build-webkit --gtk --release --makeargs='-j32'
  Building webkit
  Running in sandbox: "flatpak" "build" "--die-with-parent" "--bind-mount=/run/shm=/dev/shm"
     "--bind-mount=/etc/perl=/home/aperez/WebKit/WebKitBuild/GTK/FlatpakTreeRelease/files/lib/perl"
     "--bind-mount=/run/host//tmp=/tmp" "--bind-mount=/app/webkit=/home/aperez/WebKit"
     "--bind-mount=/app/webkit/WebKitBuild/Release=/home/aperez/WebKit/WebKitBuild/GTK/Release"
     "--env=LANG=en_US.utf8" "--env=WEBKIT_TOP_LEVEL=/app/"
     "--env=TEST_RUNNER_INJECTED_BUNDLE_FILENAME=/app/webkit/lib/libTestRunnerInjectedBundle.so"
     "--share=ipc" "--socket=x11" "--socket=wayland" "--device=all" "--share=network"
     "--socket=pulseaudio" "--system-talk-name=org.freedesktop.GeoClue2" "--filesystem=host"
     "--socket=system-bus" "--talk-name=org.freedesktop.Flatpak"
     "--env=GST_PRESET_PATH=/app/share/gstreamer-1.0/presets/"
     "/home/aperez/WebKit/WebKitBuild/GTK/FlatpakTreeRelease"
     "/app/webkit/Tools/Scripts/build-webkit" "--release" "--gtk" "--prefix=/app"

  +  cmake --build /app/webkit/WebKitBuild/Release --config Release -- 
  ...

The same happens with “--help” (so it's impossible to read the output from
“build-webkit --help” when Flatpak is in use), and feature flag options like
“--web-rtc” (nor the counterparts like “--no-web-rtc”), which are never
passed down to the instance that runs inside the sandbox.