sandbox: only pull images if not present

[soulteary]

At present, the program will try to download the latest version of the execution image every time it is started.
Since we will definitely execute the image parameters during execution, we can directly start the program after checking the existence of the image without repeating the download.

I tried updating the code on GitHub, but nothing changed on Gerrit, so I resubmitted this PR, previous discussion:
https://go-review.googlesource.com/c/playground/+/414334/1..2

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This PR (HEAD: db57ba9) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/playground/+/420936 to see it.

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[gopherbot]

Message from Jenny Rakoczy:

Patch Set 1: Run-TryBot+1

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Message from Gopher Robot:

Patch Set 1:

(1 comment)

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Message from Gopher Robot:

Patch Set 1: TryBot-Result+1

(1 comment)

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