Allow storing the registry password in a file

[kcolford]

This lets you mount the registry password in a secret that is kept separate from regular configuration. Also it means that the password can be updated dynamically without restarting the process.

[kcolford]

this is what #348 was supposed to be but that ended up just being an empty PR due a bot automatically updating my fork of this repo

[Anton-Kalpakchiev]

In Go, it's preferred to typecast by calling T(v), where T is the new type and v is the value:
return basic.Username, string(passwordBytes)

[Anton-Kalpakchiev]

Hi Kieran,
I apologize for the delayed review. I realize there were no tests for the file before you edited it but we want to change this. Can you add a test for the function you're editing?

[gkeesh7]

@kcolford Can you please address the comments to take this PR forward.

Thank you.

[kcolford]

@gkeesh7 I'm not sure how you expect these functions to be tested... all this change does is add a little bit of extra code to fetch the secret from a file instead of inline, it feels like one of those situations that's too obvious to test

[Anton-Kalpakchiev]

Hi @kcolford, I'd suggest writing a unit test where you create a credentialStore with appropriate values. Then you run the function and assert that its return values are as expected.

To test the specific case where the registry file is read/missing/etc., you can use the os package's CreateTemp function to create a temporary file, which you can then fill with whatever values the test requires. The test would delete the file at the end.