From the looks of it the 5.x docker images don't have oss builds, you're correct. I've reached out to @jarpy who is the primary contributor to https://github.com/elastic/kibana-docker to see if there are plans to create oss-specific images for 5.x. In the meantime, perhaps you could use that repo to figure out how to make your own Kibana-5.x-oss image. Will let you know when I hear back.
We didn't ship the 5.X images as OSS only "flavours". You are welcome to either remove X-Pack, create your own image, or simply register for a free Basic license for X-Pack to get access to the features in there
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