Missing docker image for qiime2/core:2018.4

[thermokarst]

Hi @hanh1985 - please see the following in the 2018.4 Release announcement:

@q2d2

We will provide official conda builds on release day for native QIIME 2 installations. Virtual machine builds such as AWS and Docker will be released within the following week. We will post a follow-up announcement here when those builds become available.

QIIME 2 2018.4 was release on April 26th - we will have the VMs posted by May 3rd. Thanks!