Add CMD statement

Change-Id: I40ff1b5beba68ccff772b51683387220a6ae2605

Author: TristanCacqueray

Containerfile/Statement.dhall

@@ -7,6 +7,7 @@ in    < From : Text
       | Env : Prelude.Map.Type Text Text
       | Comment : Text
       | Run : Text
+      | Cmd : List Text
       | Exec : List Text
       | Workdir : Text
       | Entrypoint : List Text

Containerfile/render/Statement.dhall

@@ -16,6 +16,7 @@ let renderStatement =
           , Comment = prefixText "#"
           , Workdir = prefixText "WORKDIR"
           , Run = prefixText "RUN"
+          , Cmd = prefixTextList "CMD"
           , Exec = prefixTextList "RUN"
           , Entrypoint = prefixTextList "ENTRYPOINT"
           , Env = ./Env.dhall

README.md

@@ -37,6 +37,7 @@ ENTRYPOINT ["emacs"]
 The package implements the [Containerfile reference][ref] with these changes:
 
 * `Exec` is the `RUN` exec form
+* `Cmd` only support the preferred exec form
 * `Empty` denote an empty lines
 
 
@@ -51,6 +52,7 @@ in    < From : Text
       | Env : Prelude.Map.Type Text Text
       | Comment : Text
       | Run : Text
+      | Cmd : List Text
       | Exec : List Text
       | Workdir : Text
       | Entrypoint : List Text