imgs/base-alpine: fix mount of /dev/shm

The Alpine Linux creates /dev/shm using init script devfs but that is
not run in container. It won't even run correctly in container. That
makes /dev/shm unavailable but we need it in some cases and thus this is
hacky script that adds just very small init script that creates it.

imgs/base-alpine.sh

@@ -7,10 +7,12 @@
 ##################################################################################
 set -e
 
-# Make wait scripts executable
+# Make wait and init scripts executable
 for script in wait4boot wait4network; do
 	chmod +x "/bin/$script"
 done
+chmod +x "/etc/init.d/devshm"
+rc-update add devshm boot
 
 wait4boot
 

imgs/base-alpine/etc/init.d/devshm

+#!/sbin/openrc-run
+
+start() {
+	mkdir -p /dev/shm
+	mount -n -t tmpfs -o noexec,nosuid,nodev,mode=1777 shm /dev/shm
+}