[Dockerfile] Fix apt repo trusted key import process
[Dockerfile] Fix apt repo trusted key import process

 - It makes no sense to do `apt-get install` before `apt-get update`.
 - We actually don't need to download additional "wget" package to help
   fetch the public key, but can just use "gpg" to import the key.
 - So we can prevent "wget" install fail, and save some disk space from
   "wget" and the key file.

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