The 17.09 and 17.06 channels of the Docker snap have been unmaintained for quite some time now, and frankly anyone still relying on them should seriously consider upgrading (a while ago). 
As such, I’m going to officially close them, which functionally means that anyone still using them will no longer receive any updates (although “no longer receiving updates” has been true on those channels for quite some time anyhow…) – I’m told by @ijohnson that snap refresh docker --channel=latest or snap switch docker --channel=latest is probably the easiest way to get back to the supported stable channel (so please do so!). 
All that being said, I’ve also got 19.03.8 in candidate / beta / edge ATM, and it could use some explicit testing before I push it into stable! 