Update Guide For zsh users(gradle dockerizing not working)

In my environment(Mac OS BigSur, Gradle, zsh).

This command not worked for me:

```
docker build --build-arg JAR_FILE=build/libs/*.jar -t springio/gs-spring-boot-docker .
```

And it worked for me:

```
docker build --build-arg 'JAR_FILE=build/libs/*.jar' -t springio/gs-spring-boot-docker .
```