Strange STDOUT Issue With Hooks

Reproduce with:
https://github.com/jamesward/springboot-kotlin-reactive-demo/tree/repro-stdout

When the build hook command runs it stops outputting after a few seconds, like:
```
[ ! ] Running command: ./gradlew --no-daemon --console=plain bootBuildImage --imageName=$IMAGE_URL
+ ./gradlew --no-daemon --console=plain bootBuildImage --imageName=gcr.io/jw-demo/springboot-kotlin-reactive-demo
Downloading https://services.gradle.org/distributions/gradle-6.7-bin.zip
.........10%!(NOVERB)
```

If I run the same command in Cloud Shell (not via Button) I get:
```
./gradlew --no-daemon --console=plain bootBuildImage --imageName=gcr.io/jw-de
mo/springboot-kotlin-reactive-demo
Downloading https://services.gradle.org/distributions/gradle-6.7-bin.zip
.........10%..........20%..........30%..........40%..........50%.........60%..........70%..........80%..........90%..........100
%
Welcome to Gradle 6.7!
...
```