docker: add entrypoint script

Entrypoint script starts dnsmasq before shell is launched.

Dockerfile

@@ -13,6 +13,9 @@ ENV PATH /system/bin
 # Copy libc, linker and few utilities.
 COPY /system/$SYSTEM_TYPE /system
 
+# Copy entrypoint script.
+COPY /entrypoint.sh /entrypoint.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
 RUN busybox mkdir -p /data/data/com.termux/files && \
@@ -84,4 +87,5 @@ WORKDIR /data/data/com.termux/files/home
 SHELL ["/data/data/com.termux/files/usr/bin/sh", "-c"]
 USER 1000:1000
 
+ENTRYPOINT ["/entrypoint.sh"]
 CMD ["/data/data/com.termux/files/usr/bin/login"]

entrypoint.sh

@@ -0,0 +1,21 @@
+#!/system/bin/sh
+
+if [ "$(id -u)" = "0" ] && [ -z "$(/system/bin/busybox pidof dnsmasq)" ]; then
+	/system/bin/sh -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
+else
+	echo "[!] Container is running as non-root, unable to start dnsmasq. DNS will be unavailable." >&2
+fi
+
+if [ $# -ge 1 ]; then
+	exec "$@"
+else
+	if [ "$(id -u)" = "0" ]; then
+		exec /system/bin/su - system -c "/data/data/com.termux/files/usr/bin/login"
+	else
+		exec /data/data/com.termux/files/usr/bin/login
+	fi
+fi