[SPARK-50588][BUILD] build-docs skips building R docs on host when …

…`SKIP_RDOC=1`

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

`build-docs` skips calling `bundle` command when `SKIP_RDOC=1`, so that devs do not need to install `bundle` on the host.

### Why are the changes needed?

Skip unnecessary steps when `SKIP_RDOC=1`

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

No.

### How was this patch tested?

Manually test.

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

No.

Closes apache#49200 from pan3793/SPARK-50588.

Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

dev/spark-test-image-util/docs/build-docs

@@ -49,20 +49,22 @@ docker run \
   --interactive --tty "${IMG_URL}" \
   /bin/bash -c "sh ${BUILD_DOCS_SCRIPT_PATH}"
 
-# 4.Build docs on host: `r doc`.
-#
-# Why does `r` document need to be compiled outside the container?
-# Because when compiling inside the container, the permission of the directory
-# `/__w/spark/spark/R/pkg/docs` automatically generated by `RScript` is `dr-xr--r-x`,
-# and when writing to subsequent files, will throw an error as:
-# `! [EACCES] Failed to copy '/usr/local/lib/R/site-library/pkgdown/BS5/assets/katex-auto.js'
-# to '/__w/spark/spark/R/pkg/docs/katex-auto.js': permission denied`
-export SKIP_ERRORDOC=1
-export SKIP_SCALADOC=1
-export SKIP_PYTHONDOC=1
-export SKIP_SQLDOC=1
-cd docs
-bundle exec jekyll build
+if [[ "$SKIP_RDOC" != "1" ]]; then
+  # 4.Build docs on host: `r doc`.
+  #
+  # Why does `r` document need to be compiled outside the container?
+  # Because when compiling inside the container, the permission of the directory
+  # `/__w/spark/spark/R/pkg/docs` automatically generated by `RScript` is `dr-xr--r-x`,
+  # and when writing to subsequent files, will throw an error as:
+  # `! [EACCES] Failed to copy '/usr/local/lib/R/site-library/pkgdown/BS5/assets/katex-auto.js'
+  # to '/__w/spark/spark/R/pkg/docs/katex-auto.js': permission denied`
+  export SKIP_ERRORDOC=1
+  export SKIP_SCALADOC=1
+  export SKIP_PYTHONDOC=1
+  export SKIP_SQLDOC=1
+  cd docs
+  bundle exec jekyll build
+fi
 
 # 5.Remove container image.
 IMG_ID=$(docker images | grep "${IMG_TAG}" | awk '{print $3}')