Ubuntu – “Package ‘docker-ce’ has no installation candidate” in 18.04

I'm following the official Docker installation guide for docker-ce on Ubuntu.

When I get to sudo apt install docker-ce I get E: Package 'docker-ce' has no installation candidate:

$ sudo apt-get update
Hit:1 http://gb.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://gb.archive.ubuntu.com/ubuntu bionic-updates InRelease
Hit:3 http://gb.archive.ubuntu.com/ubuntu bionic-backports InRelease
Hit:4 http://security.ubuntu.com/ubuntu bionic-security InRelease
Reading package lists... Done

$ sudo apt-get install \
>     apt-transport-https \
>     ca-certificates \
>     curl \
>     software-properties-common
Reading package lists... Done
Building dependency tree       
Reading state information... Done
ca-certificates is already the newest version (20180409).
curl is already the newest version (7.58.0-2ubuntu3).
software-properties-common is already the newest version (0.96.24.32.1).
The following NEW packages will be installed
  apt-transport-https
0 to upgrade, 1 to newly install, 0 to remove and 0 not to upgrade.
Need to get 1,692 B of archives.
After this operation, 152 kB of additional disk space will be used.
Do you want to continue? [Y/n] 
Get:1 http://gb.archive.ubuntu.com/ubuntu bionic/universe amd64 apt-transport-https all 1.6.1 [1,692 B]
Fetched 1,692 B in 0s (65.4 kB/s)               
Selecting previously unselected package apt-transport-https.
(Reading database ... 116694 files and directories currently installed.)
Preparing to unpack .../apt-transport-https_1.6.1_all.deb ...
Unpacking apt-transport-https (1.6.1) ...
Setting up apt-transport-https (1.6.1) ...

$ curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
OK

$ sudo apt-key fingerprint 0EBFCD88
pub   rsa4096 2017-02-22 [SCEA]
      9DC8 5822 9FC7 DD38 854A  E2D8 8D81 803C 0EBF CD88
uid           [ unknown] Docker Release (CE deb) <docker@docker.com>
sub   rsa4096 2017-02-22 [S]

$ sudo add-apt-repository \
>    "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
>    $(lsb_release -cs) \
>    stable"
Hit:1 http://gb.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://gb.archive.ubuntu.com/ubuntu bionic-updates InRelease                                                                                    
Hit:3 http://gb.archive.ubuntu.com/ubuntu bionic-backports InRelease                                                                                  
Get:4 https://download.docker.com/linux/ubuntu bionic InRelease [64.4 kB]                                                       
Hit:5 http://security.ubuntu.com/ubuntu bionic-security InRelease                                                               
Fetched 64.4 kB in 0s (141 kB/s)
Reading package lists... Done

robin@xps:~/git/snapcraft.io$ sudo apt-get update
Hit:1 http://gb.archive.ubuntu.com/ubuntu bionic InRelease
Hit:2 http://gb.archive.ubuntu.com/ubuntu bionic-updates InRelease                                      
Hit:3 http://gb.archive.ubuntu.com/ubuntu bionic-backports InRelease                                                                                   
Hit:4 https://download.docker.com/linux/ubuntu bionic InRelease                                                                                        
Hit:5 http://security.ubuntu.com/ubuntu bionic-security InRelease                                                                
Reading package lists... Done  

$ sudo apt-get install docker-ce
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Package docker-ce is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'docker-ce' has no installation candidate

My first thought was that Docker haven't released a package for Bionic yet (it's very new) but when I searched for "install docker-ce ubuntu 18.04" I found a guide which seems 18.04 specific, and basically just gives exactly the same instructions – suggesting that it at least worked for the author:

https://linuxconfig.org/how-to-install-docker-on-ubuntu-18-04-bionic-beaver

Does anyone have any idea why this might not be working for me?