Version 18.09.1+dfsg1-5 of docker.io is marked for autoremoval from testing on Thu 18 Apr 2019. It depends (transitively) on golang-github-ncw-swift, notary, affected by #923804, #924119. You should try to prevent the removal by fixing these RC bugs.
The current maintainer is looking for someone who can help with
the maintenance of this package. If you are interested in this
package, please consider helping out. One way you can help is
offer to be a co-maintainer or triage bugs in the BTS. Please see bug number #908868 for more information.