Add a PPA with key completely manually, without `add-apt-repository` OR `apt-key adv --recv-keys`

I need the exact and complete series of shell incantations for adding a PPA with key in the following situation:

• add-apt-repository is not installed
• gnupg and gnupg2 are not installed, making it impossible to use apt-key adv --recv-keys
• installing those missing tools beforehand is to be avoided at all cost because in the context of docker image building it would require another apt-get update run which is slooooowwwwwwww

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