[SPARK-25803][K8S] Fix docker-image-tool.sh -n option

## What changes were proposed in this pull request?

docker-image-tool.sh uses getopts in which a colon signifies that an
option takes an argument.  Since -n does not take an argument it
should not have a colon.

## How was this patch tested?

Following the reproduction in [JIRA](https://issues.apache.org/jira/browse/SPARK-25803):-

0. Created a custom Dockerfile to use for the spark-r container image.
In each of the steps below the path to this Dockerfile is passed with the '-R' option.
(spark-r is used here simply as an example, the bug applies to all options)

1. Built container images without '-n'.
The [result](https://gist.github.com/sel/59f0911bb1a6a485c2487cf7ca770f9d) is that the '-R' option is honoured and the hello-world image is built for spark-r, as expected.

2. Built container images with '-n' to reproduce the issue
The [result](https://gist.github.com/sel/e5cabb9f3bdad5d087349e7fbed75141) is that the '-R' option is ignored and the default container image for spark-r is built

3. Applied the patch and re-built container images with '-n' and did not reproduce the issue
The [result](https://gist.github.com/sel/6af14b95012ba8ff267a4fce6e3bd3bf) is that the '-R' option is honoured and the hello-world image is built for spark-r, as expected.

Closes #22798 from sel/fix-docker-image-tool-nocache.

Authored-by: Steve <[email protected]>
Signed-off-by: Marcelo Vanzin <[email protected]>
(cherry picked from commit 9b98d91)
Signed-off-by: Marcelo Vanzin <[email protected]>

Author: sel

bin/docker-image-tool.sh

@@ -145,7 +145,7 @@ PYDOCKERFILE=
 RDOCKERFILE=
 NOCACHEARG=
 BUILD_PARAMS=
-while getopts f:p:R:mr:t:n:b: option
+while getopts f:p:R:mr:t:nb: option
 do
  case "${option}"
  in