Fix mysql repo key issues

[hcooper]

The way apt handles signing keys has totally changed since the base image of this docker container was built, causing apt-update to fail, and hence container building to fail.

hcooper@rw:/containers/ropewiki/git/app$ docker run -it --network host mysql:5.7-debian bash
root@booty:/# apt update
[...]
Err:5 http://repo.mysql.com/apt/debian buster InRelease
  The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7B3B788A8D3785C
[...]
Reading package lists... Done
W: GPG error: http://repo.mysql.com/apt/debian buster InRelease: The following signatures couldn't be verified
    because the public key is not available: NO_PUBKEY B7B3B788A8D3785C
E: The repository 'http://repo.mysql.com/apt/debian buster InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

The fix is to use gpg to fetch and export the key to /etc/apt/keyrings/. Works fine now.

Another reason we should be upgrading the core software stack 😉 cough #55 cough