docker: re-enable passing environment variables to the build

The -E flag for sudo has previously been removed to work around some
strange issues.
Re-add the safer --preserve-envs flag listing all the settings currently
available.

This allows in particular to override the serdes configuration again
without editing the runme.sh file - e.g.:
docker run -i -t -v "$PWD":/work -e SERDES=8S_9_2 lx2160a_build -u 0 -g 0
Signed-off-by: Josua Mayer <josua@solid-run.com>

docker/entry.sh

@@ -33,5 +33,10 @@ sudo -u $user git config --global user.name "LX2160A Toolchain Container"
 sudo -u $user git config --global user.email "support@solid-run.com"
 
 cd /work
-# now run the build script as the build user
-sudo -u $user ./runme.sh
+# now run the build script as the build user, preserving config variables
+sudo \
+	--preserve-env=RELEASE,DDR_SPEED,SERDES,UEFI_RELEASE \
+	--preserve-env=SHALLOW,SECURE,ATF_DEBUG,DISTRO \
+	--preserve-env=BR2_PRIMARY_SITE \
+	-u $user \
+	./runme.sh