Do not validate variable substitutions in CMD and ENTRYPOINT instructions

[rcjsuen]

This is an extension to #60 for CMD and ENTRYPOINT instructions reported by microsoft/vscode-docker#2738. We need skip validation of variable substitutions in CMD and ENTRYPOINT instructions as can be dependent on the shell as well.

#escape=`
FROM mcr.microsoft.com/windows/servercore:ltsc2019
ENTRYPOINT powershell -Command `
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}

CMD powershell -Command `
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}

$ dockerfile-utils lint Dockerfile 
Line: 4
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                       ^
Error: failed to process "${env:address}": unsupported modifier (a) in substitution

Line: 4
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                          ^
Error: failed to process "${env:podname}": unsupported modifier (p) in substitution

Line: 4
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                         ^
Error: failed to process "${env:source}": unsupported modifier (s) in substitution

Line: 4
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                                       ^
Error: failed to process "${env:dest}": unsupported modifier (d) in substitution

Line: 4
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                                                      ^
Error: failed to process "${env:namespace}": unsupported modifier (n) in substitution

Line: 7
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                       ^
Error: failed to process "${env:address}": unsupported modifier (a) in substitution

Line: 7
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                          ^
Error: failed to process "${env:podname}": unsupported modifier (p) in substitution

Line: 7
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                         ^
Error: failed to process "${env:source}": unsupported modifier (s) in substitution

Line: 7
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                                       ^
Error: failed to process "${env:dest}": unsupported modifier (d) in substitution

Line: 7
  kubectl port-forward --address ${env:address} pod/${env:podname} ${env:source}:${env:dest} -n ${env:namespace}
                                                                                                      ^
Error: failed to process "${env:namespace}": unsupported modifier (n) in substitution