only trigger by container events

[lalyos]

Docker introduced new type of events, which are breaking dockerhook.
For example in case of an image type event (like pull) the inspec() function will fail, as it was meant to inspect containers.

2019/11/24 13:51:51 [HACK] msg: &docker.APIEvents{Action:"pull", Type:"image", Actor:docker.APIActor{ID:"alpine:3.7", Attributes:map[stri$
g]string{"name":"alpine"}}, Status:"pull", ID:"alpine:3.7", From:"", Time:1574599911, TimeNano:1574599911309698300} 
panic: runtime error: slice bounds out of range

goroutine 123 [running]:
main.trigger(0xc0000a6340, 0x1, 0x1, 0xc00044d348, 0x4, 0xc00044d360, 0xa, 0xc0005ce000)
        /Users/lalyos/go/src/github.com/progrium/dockerhook/dockerhook.go:79 +0x69e
created by main.main
        /Users/lalyos/go/src/github.com/progrium/dockerhook/dockerhook.go:133 +0x6c4