spec: add workingDirectory to image manifest
spec: add workingDirectory to image manifest

`workingDirectory` was added in 1d45bd9 but not fully specified. This
adds details to the ImageManifest definition, and also mandates that the
ACE must fail if the directory does not exist within the rendered
layout.

endocode/appc-spec,dgonyeo/spec,marineam/spec,appc/spec,3ofcoins/appc-spec,dustymabe/spec,marineam/spec,appc/spec,stufflebear/spec,marineam/spec,yifan-gu/spec,zenlinTechnofreak/spec,iaguis/appc-spec,zenlinTechnofreak/spec,jonboulle/spec,dgonyeo/spec,ChengTiesheng/spec,ChengTiesheng/spec,yifan-gu/spec,yifan-gu/spec,dgonyeo/spec,krnowak/appc-spec,stufflebear/spec,craimbert/spec,squeed/spec,tklauser/spec,tklauser/spec,3ofcoins/appc-spec,stufflebear/spec,squeed/spec,squeed/spec,jonboulle/spec,appc/spec,ChengTiesheng/spec,craimbert/spec,apcera/appc-spec,jonboulle/spec,craimbert/spec,iaguis/appc-spec,tklauser/spec,endocode/appc-spec,sgotti/spec,alban/appc-spec,apcera/appc-spec,alban/appc-spec,apcera/appc-spec,alban/appc-spec,dustymabe/spec,endocode/appc-spec,krnowak/appc-spec,iaguis/appc-spec,zenlinTechnofreak/spec,krnowak/appc-spec,dustymabe/spec,3ofcoins/appc-spec,sgotti/spec