The latest image builds (tags) for Feb 2019 releases do not appear on dockerhub yet - for example 5.15.2 released 18 Feb 2019 (but check other releases too)
$ docker pull atlassian/bitbucket-server:5.15.2
Error response from daemon: manifest for atlassian/bitbucket-server:5.15.2 not found
$ docker pull atlassian/bitbucket-server:5.15.1
5.15.1: Pulling from atlassian/bitbucket-server
Digest: sha256:4f282e66ea263c1f31f28eafebe640f5acb3d6bd821ee71f71e2727cf77876b2
Status: Image is up to date for atlassian/bitbucket-server:5.15.1
