Dockerfile.gcc10: Add flac, vorbis

Dockerfile.gcc10

@@ -120,6 +120,17 @@ RUN set -eux; \
     make clean-junk; \
     rm -rf pkg/*
 
+# Auxiliary packages required by 'non-standard' cores
+ENV AUX_MXE_PACKAGES \
+    flac \
+    vorbis
+
+RUN set -eux; \
+    cd /usr/lib/mxe; \
+    make $AUX_MXE_PACKAGES; \
+    make clean-junk; \
+    rm -rf pkg/*
+
 RUN echo "developer:developer" | chpasswd && adduser developer sudo
 
 ENV HOME=/developer