[SPARK-27626][K8S] Fix docker-image-tool.sh to be robust in non-bash shell env

## What changes were proposed in this pull request?

Although we use shebang `#!/usr/bin/env bash`, `minikube docker-env` returns invalid commands in `non-bash` environment and causes failures at `eval` because it only recognizes the default shell. We had better add `--shell bash` option explicitly in our `bash` script.

```bash
$ bash -c 'eval $(minikube docker-env)'
bash: line 0: set: -g: invalid option
set: usage: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
bash: line 0: set: -g: invalid option
set: usage: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
bash: line 0: set: -g: invalid option
set: usage: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]
bash: line 0: set: -g: invalid option
set: usage: set [-abefhkmnptuvxBCHP] [-o option-name] [--] [arg ...]

$ bash -c 'eval $(minikube docker-env --shell bash)'
```

## How was this patch tested?

Manual. Run the script with non-bash shell environment.
```
bin/docker-image-tool.sh -m -t testing build
```

Closes apache#24517 from dongjoon-hyun/SPARK-27626.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

bin/docker-image-tool.sh

@@ -282,7 +282,7 @@ do
    if ! minikube status 1>/dev/null; then
      error "Cannot contact minikube. Make sure it's running."
    fi
-   eval $(minikube docker-env)
+   eval $(minikube docker-env --shell bash)
    ;;
   u) SPARK_UID=${OPTARG};;
  esac