docs(troubleshooting.md): Fixed alpine install example

[aos]

First of all, thank you for your wonderful documentation on installing for Docker/Alpine. I was able to dockerize my simple script using parts of the example Dockerfile you gave. It was missing one thing though:

We chown /app directory but I got the error that it doesn't exist yet. I added it in the mkdir command and that fixed the problem.

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[aos]

I signed the CLA

[googlebot]

CLAs look good, thanks!

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[aslushnikov]

Awesome, thanks!