add --init to suggested docker run invocations

See #2096; this should fix unreaped zombies when using an auth-script or
ip-check-script that spawns its own subprocesses, then exits before reaping
them.

distrib/docker/README.md

 expose the ports:
 
 ```shell
-docker run --name ergo -d -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
+docker run --init --name ergo -d -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
 ```
 
 This will start Ergo and listen on ports 6667 (plain text) and 6697 (TLS).
 
 ```shell
 docker volume create ergo-data
-docker run -d -v ergo-data:/ircd -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
+docker run --init -d -v ergo-data:/ircd -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
 ```
 
 Or to mount a folder from your host machine:
 
 ```shell
 mkdir ergo-data
-docker run -d -v $(PWD)/ergo-data:/ircd -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
+docker run --init -d -v $(PWD)/ergo-data:/ircd -p 6667:6667 -p 6697:6697 ghcr.io/ergochat/ergo:stable
 ```
 
 ## Customising the config