GPG key check in dockerfile broken (?), removed

[jotelha]

Was not able to build Docker image without removing the key key check. Keyserver ha.pool.sks-keyservers.net yields No results found: No keys found.

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This change is 

[bwitt]

I think it's still a good idea to validate the downloaded files; I notice the newer versions e.g. https://github.com/krallin/tini/releases/tag/v0.19.0 have sha256sums we could validate instead of using gpg

[bwitt]

oh actually, we could just verify the current version file now via sha256sum with a hardcoded sha256 instead of gpg e.g.

&& echo '<output of sha256sum> /usr/local/bin/tini' | sha256sum --check 

[dungmv]

consider changing keyserver https://stackoverflow.com/questions/66217436/gpg-keyserver-receive-failed-no-name
after changing keyserver to keyserver.ubuntu.com, I was build image success

[BlackthornYugen]

@jotelha #804 should have corrected this. Can you verify if the updated dockerfile works for you?

[BlackthornYugen]

I'll close this for now. If there's still an issue we can use #842 to track it.