Apt-key is deprecated in 20.20

File: [engine/install/ubuntu.md](https://docs.docker.com/engine/install/ubuntu/)

```
curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
```

Should be replaced with something like

```
wget https://download.docker.com/linux/ubuntu/gpg -O /etc/apt/trusted.gpg.d/docker.gpg
```