gentoo: use the offical images, tweak config

gentoo

-FROM ksmanis/portage:latest as portage
-FROM ksmanis/stage3:amd64-nomultilib
+FROM gentoo/portage:latest as portage
+FROM gentoo/stage3:latest
 
 COPY --from=portage /var/db/repos/gentoo /var/db/repos/gentoo
 
 ENV ACCEPT_KEYWORDS="amd64" \
     EMERGE_DEFAULT_OPTS="--jobs=9 --load-average=8 --with-bdeps=y" \
-    FEATURES="ccache -sandbox -usersandbox -ipc-sandbox -network-sandbox -pid-sandbox" \
+    FEATURES="ccache -sandbox -usersandbox -ipc-sandbox -mount-sandbox -network-sandbox -pid-sandbox" \
     CCACHE_SIZE="2G" \
     MAKEOPTS="-j9 -l8" \
-    USE="cairo introspection jpeg tiff -python_targets_python2_7"
+    USE="cairo introspection jpeg tiff"
 
 RUN emerge dev-util/ccache dev-lang/rust-bin \
 &&  emerge app-text/poppler \