docker: add alternate entrypoint script

This one spawns a root shell.

Dockerfile

@@ -15,6 +15,7 @@ COPY /system/$SYSTEM_TYPE /system
 
 # Copy entrypoint script.
 COPY /entrypoint.sh /entrypoint.sh
+COPY /entrypoint_root.sh /entrypoint_root.sh
 
 # Extract bootstrap archive and create symlinks.
 ADD https://github.com/termux/termux-packages/releases/download/bootstrap-$BOOTSTRAP_VERSION/bootstrap-$BOOTSTRAP_ARCH.zip /bootstrap.zip

entrypoint_root.sh

@@ -0,0 +1,20 @@
+#!/system/bin/sh
+
+if [ "$(id -u)" != "0" ]; then
+	echo "Failure: /entrypoint_root.sh must be started as root." >&2
+	exit 1
+fi
+
+if [ -z "$(/system/bin/busybox pidof dnsmasq)" ]; then
+	/system/bin/mksh -T /dev/ptmx -c "/system/bin/dnsmasq -u root -g root --pid-file /dnsmasq.pid" >/dev/null 2>&1
+	sleep 1
+	if [ -z "$(/system/bin/busybox pidof dnsmasq)" ]; then
+		echo "[!] Failed to start dnsmasq, host name resolution may fail." >&2
+	fi
+fi
+
+if [ $# -ge 1 ]; then
+	exec "$@"
+else
+	exec /data/data/com.termux/files/usr/bin/login
+fi