Docker health checks for services

gistfile1.txt

```
  healthcheck:
    test: ["CMD-SHELL", "[ $$(expr $$(date +%s) - $$(stat -c '%Y' \"/app/health_check\")) -lt 60 ] || exit 1"  
    interval: 60s
    timeout: 5s
    retries: 3
```
This checks the last modified date of a file `/app/health_check` which the service should update
whenever it wants to mark itself as "alive". This command will check that it was updated in the
last 60 seconds and exit with a zero code. If its more than 60 seconds, it will exit with 1.

The command is actually this:
`[ $(expr $(date +%s) - $(stat -c '%Y' "/app/health_check")) -lt 60 ] || exit 1`
but the `$` and `"` are escaped.