boot watch sometimes stops notifying for filesystem changes

We're running into issues where sometimes `boot watch` seemingly stops working and doesn't run the build pipeline any more after file changes. After restarting the boot instance it works again as expected.

I enabled the `:verbose` flag for the `watch` task and confirmed that it doesn't print anything when a file changes when I run into the issue. When starting a new boot instance in parallel, like `boot --source-paths src watch show -f)` this one does react to file changes, while the other one doesn't.

I also connected to the boot instance having the issue with nrepl, started a watch task in there with `(boot (watch :verbose true) (show :fileset true))`. This also doesn't react to file changes.

Any clue how I can debug what is going on?

Some info about the env this is happing:

boot -V:
```
#http://boot-clj.com
#Wed Apr 19 07:39:27 UTC 2017
BOOT_CLOJURE_NAME=org.clojure/clojure
BOOT_CLOJURE_VERSION=1.9.0-alpha15
BOOT_VERSION=2.7.1
```

JVM: `OpenJDK 64-Bit Server VM 1.8.0_121-8u121-b13-1~bpo8+1-b13`

I don't know if this matters, but for completeness:

```
cat /proc/version
Linux version 4.9.13-moby (root@11fbdc1f630f) (gcc version 6.2.1 20160822 (Alpine 6.2.1) ) #1 SMP Tue Mar 21 21:21:05 UTC 2017
```
running in Docker `17.03.1-ce-rc1`.