[SPARK-48899][K8S] Fix ENV key value format in K8s Dockerfiles

### What changes were proposed in this pull request?

This PR aims to fix `ENV` key value format in K8s Dockerfiles.

### Why are the changes needed?

To follow the Docker guideline to fix the following legacy format.
- https://docs.docker.com/reference/build-checks/legacy-key-value-format/
```
- LegacyKeyValueFormat: "ENV key=value" should be used instead of legacy "ENV key value" format
```

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Pass the CIs.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes #47357 from dongjoon-hyun/SPARK-48899.

Authored-by: Dongjoon Hyun <dhyun@apple.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
(cherry picked from commit 40899c1)
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

Author: dongjoon-hyun

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/Dockerfile

@@ -52,7 +52,7 @@ COPY examples /opt/spark/examples
 COPY kubernetes/tests /opt/spark/tests
 COPY data /opt/spark/data
 
-ENV SPARK_HOME /opt/spark
+ENV SPARK_HOME=/opt/spark
 
 WORKDIR /opt/spark/work-dir
 RUN chmod g+w /opt/spark/work-dir

resource-managers/kubernetes/docker/src/main/dockerfiles/spark/bindings/R/Dockerfile

@@ -32,7 +32,7 @@ RUN \
   rm -rf /var/cache/apt/* && rm -rf /var/lib/apt/lists/*
 
 COPY R ${SPARK_HOME}/R
-ENV R_HOME /usr/lib/R
+ENV R_HOME=/usr/lib/R
 
 WORKDIR /opt/spark/work-dir
 ENTRYPOINT [ "/opt/entrypoint.sh" ]