package/docker-engine: add wrapper script for logging to syslog

Dockerd logs only to stdout/stderr [1], which is lost with
--background. The upstream SysV init script [2] logs to a file by
passing --no-close to start-stop-daemon and redirecting the output,
but that option is not supported by Busybox' start-stop-daemon.

The wrapper script added with this commit captures the output of
dockerd (or whatever other command it is given) and forwards each line
to syslog.

[1] https://github.com/moby/moby/discussions/48260
[2] https://github.com/moby/moby/blob/50c3d19179e69f9e7ff01f688c4dbf32c5129ced/contrib/init/sysvinit-debian/docker

Signed-off-by: Fiona Klute (WIWA) <fiona.klute@gmx.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/docker-engine/S60dockerd

@@ -10,10 +10,14 @@ DOCKERD_ARGS=""
 
 start() {
 	printf 'Starting %s: ' "$DAEMON"
+	# Dockerd logs only to stdout/stderr, which is lost with
+	# --background. The wrapper script runs the given command
+	# (after "--", including dockerd) and forwards stdout/stderr
+	# to syslog.
 	# shellcheck disable=SC2086 # we need word splitting for DOCKERD_ARGS
 	start-stop-daemon --start --background --pidfile "$PIDFILE" \
-		--exec "/usr/bin/$DAEMON" \
-		-- --pidfile "$PIDFILE" $DOCKERD_ARGS
+		--exec /usr/libexec/dockerd-syslog-wrapper.sh \
+		-- "/usr/bin/$DAEMON" --pidfile "$PIDFILE" $DOCKERD_ARGS
 	status=$?
 	if [ "$status" -eq 0 ]; then
 		echo "OK"

package/docker-engine/docker-engine.mk

@@ -72,6 +72,8 @@ endef
 define DOCKER_ENGINE_INSTALL_INIT_SYSV
 	$(INSTALL) -D -m 755 package/docker-engine/S60dockerd \
 		$(TARGET_DIR)/etc/init.d/S60dockerd
+	$(INSTALL) -D -m 755 package/docker-engine/dockerd-syslog-wrapper.sh \
+		$(TARGET_DIR)/usr/libexec/dockerd-syslog-wrapper.sh
 endef
 
 define DOCKER_ENGINE_USERS

package/docker-engine/dockerd-syslog-wrapper.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+"${@}" 2>&1 | while read -r LINE; do
+	logger -t "$(basename "${1}")" "$LINE";
+done