## 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 <sel@users.noreply.github.com>
Signed-off-by: Marcelo Vanzin <vanzin@cloudera.com>
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