exec: "sleep 86400": executable file not found in $PATH

I must be doing something wrong here but can't figure out what.

With imagebuilder from latest head, and docker-1.13.1-51.git4032bd5.fc27.x86_64

$ imagebuilder -t foo -f Dockerfile.min .
--> FROM fedora:27 as 0
--> RUN touch /etc/foo
--> Committing changes to foo ...
--> Done
$ docker run -it --rm foo 
/usr/bin/docker-current: Error response from daemon: oci runtime error: container_linux.go:247: starting container process caused "exec: \"sleep 86400\": executable file not found in $PATH".
$ docker run -it --rm foo /bin/bash
[root@7e2cdeabd622 /]# exit
$ docker run -it --rm foo sleep 1
$

First of all, where did "sleep 86400" come from? fedora:27 has no Cmd or Entrypoint set. foo does.

$ docker run -it --rm fedora:27
/usr/bin/docker-current: Error response from daemon: No command specified.
See '/usr/bin/docker-current run --help'.
$ docker inspect foo
[
    {
        "Id": "sha256:948b2333e11c0dfbedf4bde7e7a7fe6749376dd5d5fae112992edfa600838ad8",
        "RepoTags": [
            "foo:latest"
        ],
        "RepoDigests": [],
        "Parent": "sha256:9110ae7f579f35ee0c3938696f23fe0f5fbe641738ea52eb83c2df7e9995fa17",
        "Comment": "",
        "Created": "2018-04-18T19:01:37.421678496Z",
        "Container": "fc1eb310d84b08683a2eab434268e46313bdf46b2a533606e4cdbed1a5e7727e",
        "ContainerConfig": {
            "Hostname": "fc1eb310d84b",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "DISTTAG=f27container",
                "FGC=f27",
                "FBR=f27"
            ],
            "Cmd": [
                "sleep 86400"
            ],
            "Image": "fedora:27",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": [
                "/bin/sh",
                "-c"
            ],
            "OnBuild": null,
            "Labels": {}
        },
        "DockerVersion": "1.13.1",
        "Author": "",
        "Config": {
            "Hostname": "",
            "Domainname": "",
            "User": "",
            "AttachStdin": false,
            "AttachStdout": false,
            "AttachStderr": false,
            "Tty": false,
            "OpenStdin": false,
            "StdinOnce": false,
            "Env": [
                "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
                "DISTTAG=f27container",
                "FGC=f27",
                "FBR=f27"
            ],
            "Cmd": [
                "sleep 86400"
            ],
            "Image": "",
            "Volumes": null,
            "WorkingDir": "",
            "Entrypoint": [],
            "OnBuild": null,
            "Labels": {}
        },
        "Architecture": "amd64",
        "Os": "linux",
        "Size": 235250693,
        "VirtualSize": 235250693,
        "GraphDriver": {
            "Name": "overlay2",
            "Data": {
                "LowerDir": "/var/lib/docker/overlay2/70bea816b920919e3afcdaca1f7c572c83308f4bee6379450e17acba57da28ff/diff",
                "MergedDir": "/var/lib/docker/overlay2/6e1b01b002e68b2410fe67bcfee1bc7647346563074c82bc1abc534681459b01/merged",
                "UpperDir": "/var/lib/docker/overlay2/6e1b01b002e68b2410fe67bcfee1bc7647346563074c82bc1abc534681459b01/diff",
                "WorkDir": "/var/lib/docker/overlay2/6e1b01b002e68b2410fe67bcfee1bc7647346563074c82bc1abc534681459b01/work"
            }
        },
        "RootFS": {
            "Type": "layers",
            "Layers": [
                "sha256:39bae602f7539e626f6894ecded6d12a6c56745dbab9aee940e258c766e090e8",
                "sha256:de41557ebc416b99c4bb90ae5e39f4c451055236db8aff0cbe10f8206a275039"
            ]
        }
    }
]

Second, why does it fail to run that?

Start by reproducing the behavior with imagebuilder, Dockerfile.min, and the documented docker run, docker inspect, and docker run ... sleep 1 commands. Trace where the generated Cmd and Entrypoint values come from and compare them with the base image metadata. Done means the cause of the unexpected command and failed startup is identified and covered by a regression test.