gentoo: Use stable, pull bubblewrap from testing

Due to a regression, gdk-pixbuff is currently broken in testing.
Accordingly, use stable. At the same time, bubblewrap is only available
in testing, accordingly, pull it from there.

Thanks to Poncho for debugging!

Signed-off-by: georg <georg@riseup.net>

gentoo

@@ -3,7 +3,7 @@ FROM gentoo/stage3-amd64:latest
 
 COPY --from=portage /usr/portage /usr/portage
 
-ENV ACCEPT_KEYWORDS="~amd64" \
+ENV ACCEPT_KEYWORDS="amd64" \
     CCACHE_DIR="/gentoo/ccache" \
     EMERGE_DEFAULT_OPTS="--jobs=9 --load-average=8 --with-bdeps=y" \
     FEATURES="-sandbox -usersandbox" \
@@ -18,6 +18,6 @@ RUN mkdir -p /gentoo/ccache \
     dev-util/ccache \
     media-libs/exiftool \
     media-libs/mutagen \
-    sys-apps/bubblewrap \
     virtual/ffmpeg \
-    x11-libs/gdk-pixbuf
+    x11-libs/gdk-pixbuf \
+&&  ACCEPT_KEYWORDS="~amd64" emerge sys-apps/bubblewrap