FR: set RuntimeDirectoryPreserve=yes?

### What are you trying to do?

I am sharing the tailscale socket into a caddy container via a docker volume. If I restart the tailscale process on the host (e.g on a tailscale update) The /var/run/tailscale directory is removed and thus my docker mount invalidated until I restart the caddy container. Example docker compose snippet:

```
version: "3.8"
services:
caddy:
container_name: caddy
restart: always
image: jumager/caddy
volumes:
- /var/run/docker.sock:/var/run/docker.sock
- /var/run/tailscale:/var/run/tailscale
- /run/containers:/run/containers
- ./data:/data
- ./config:/config
ports:
- 80:80
- 443:443
- 443:443/udp
- 127.0.0.1:2019:2019
environment:
- CADDY_INGRESS_NETWORKS=caddy
- CADDY_DOCKER_CADDYFILE_PATH=/config/Caddyfile
- CADDY_HOST=test.example.org
- CADDY_TAILNET_HOST=test.tailXXXXX.ts.net
- CADDY_REDIS_HOST=redis.tailXXXXX.ts.net
```

### How should we solve this?

If the tailscaled.service would set the option:

```
RuntimeDirectoryPreserve=yes
```

the /var/run/tailscale directory would persist and upon the daemon creating a new socket there the docker container would be able to reconnect to the tailscale socket without restarting the container to remount the volume.

### What is the impact of not solving this?

I have to restart my caddy container and thus disrupting any services even though I only need the tailnet in the caddy configuration for administrative services supposed to be used only from trusted tailnet users. I am using the tailscaled socket to acquire ssl certificates for services running accessible via the tailnet api, for example the watchtower API. A more detailed write up how I configure my systems can be found here:

[https://github.com/jum/caddy-docker-proxy-redis](https://github.com/jum/caddy-docker-proxy-redis)

### Anything else?

Thanks for the great software!