[MINOR][BUILD] Give run-in-container a shebang

[nchammas]

What changes were proposed in this pull request?

Give run-in-container a shebang and update its one invocation accordingly.

Why are the changes needed?

It's not common (or IMO appropriate) for a shell script to have neither an extension nor a shebang. IDEs don't know how to highlight the code, and invoking the script becomes more complicated than necessary.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

TODO:

• Local test run. (CI does not call these scripts.)

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

No.

[HyukjinKwon]

LGTM if it works

[nchammas]

@HyukjinKwon / @panbingkun - It seems this script has been broken at least since #57581 and maybe for longer.

To get it to work locally, I needed to:

• add --build-context root="${SPARK_HOME}"
• add --load
• update Python from 3.11 to 3.12

Since this doesn't run on CI, I think no one noticed the breakage.

I would prefer to remove the build-docs script entirely. The added layer of indirection + the fact that it doesn't run on CI means it will constantly be breaking for one reason or another. I also don't see the need for the script as a single Python venv under docs/ (or even at the root of the project) addresses the original motivation presented in #48860.

I won't open a PR for that, however, unless you agree. I know that some people strongly prefer to use Docker for everything. I am not one of them.