Dockerfile.gcc10: Use fixed 'known working' MXE commit, to avoid potential toolchain regressions

Dockerfile.gcc10

@@ -8,6 +8,7 @@ ENV branch=$branch
 
 ARG NUMPROC
 ARG MXE_GCC_VERSION=gcc10
+ARG MXE_COMMIT=ad07d47c3e43ec5f8d0b4cc4e5efa4e9a9a15426
 
 ENV PACKAGES \
     build-essential \
@@ -103,8 +104,8 @@ RUN set -eux; \
     cd mxe; \
     git init; \
     git remote add origin 'https://github.com/mxe/mxe.git'; \
-    git fetch --depth=1 origin master; \
-    git checkout master; \
+    git fetch --depth=1 origin ${MXE_COMMIT}; \
+    git checkout ${MXE_COMMIT}; \
     make check-requirements; \
     echo '' >> settings.mk; \
     echo 'JOBS :=' ${NUMPROC} >> settings.mk; \