Allow Args to be used in Variable Evaluation

If the Dockerfile has a `ARG CHOWN_VAL=6173:6173`, then
a `COPY --chown=${CHOWN_VAL} srcFile destFile` will fail.
However if the Dockerfile has a `ENV CHOWN_VAL=6173:6173`
then it succeeds.

The dispatchCopy() function was only going through the
list of Environment variables to resolve a Variable instead
of the list of Args and Environment variables.  If there's
a trailing equal `=` sign in the arg, assume that we'd a
variable that did not resolve and try to resolve from the
list of args.

At some point I'd like to look into converting the b.Env to
a Map from a slice like b.Args, but didn't want to run this
down now.

Fixes: containers/buildah#2192
       and probably containers/buildah#2345

Signed-off-by: TomSweeneyRedHat <[email protected]>

dispatchers.go

@@ -186,6 +186,20 @@ func dispatchCopy(b *Builder, args []string, attributes map[string]bool, flagArg
 		if err != nil {
 			return err
 		}
+		// If there's an arg ending with an '=', it's likely we did
+		// not resolve a variable value from the Environment Variables.
+		// Let's see if the variable was defined in an 'ARG' statement
+		// in the Dockerfile.
+		if strings.HasSuffix(arg, "=") {
+			var userArg []string
+			for key, value := range b.Args {
+				userArg = append(userArg, key+"="+value)
+			}
+			arg, err = ProcessWord(a, userArg)
+			if err != nil {
+				return err
+			}
+		}
 		switch {
 		case strings.HasPrefix(arg, "--chown="):
 			chown = strings.TrimPrefix(arg, "--chown=")