I'm following the official guide to install Docker.
And in step 2 Add Docker’s official GPG key, I got the following error:
$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
gpg: invalid key resource URL '/tmp/apt-key-gpghome.Rcg4Lpatvi/home:manuelschneid3r.asc.gpg'
gpg: keyblock resource '(null)': General error
gpg: key 1488EB46E192A257: 1 signature not checked due to a missing key
gpg: key 3B4FE6ACC0B21F32: 3 signatures not checked due to missing keys
gpg: key D94AA3F0EFE21092: 3 signatures not checked due to missing keys
gpg: key 871920D1991BC93C: 1 signature not checked due to a missing key
gpg: Total number processed: 9
gpg: skipped new keys: 9
How do I solve that?