Fix documentation on docker image to say you must build before running (#1987)
Fix documentation on docker image to say you must build before running (#1987)

* Can not run before building

unless you provide a library/powershell image

* Update docker.md

jsoref/PowerShell,PaulHigin/PowerShell,daxian-dbw/PowerShell,bmanikm/PowerShell,JamesWTruher/PowerShell-1,JamesWTruher/PowerShell-1,TravisEz13/PowerShell,PaulHigin/PowerShell,daxian-dbw/PowerShell,TravisEz13/PowerShell,daxian-dbw/PowerShell,bingbing8/PowerShell,bmanikm/PowerShell,TravisEz13/PowerShell,jsoref/PowerShell,PaulHigin/PowerShell,bingbing8/PowerShell,bmanikm/PowerShell,bmanikm/PowerShell,JamesWTruher/PowerShell-1,bmanikm/PowerShell,bingbing8/PowerShell,KarolKaczmarek/PowerShell,KarolKaczmarek/PowerShell,jsoref/PowerShell,KarolKaczmarek/PowerShell,bingbing8/PowerShell,KarolKaczmarek/PowerShell,KarolKaczmarek/PowerShell,jsoref/PowerShell,PaulHigin/PowerShell,jsoref/PowerShell,bingbing8/PowerShell,JamesWTruher/PowerShell-1,daxian-dbw/PowerShell,TravisEz13/PowerShell