docker compose interpolation format error now includes a hint on escaping $ characters.

Signed-off-by: Arash Deshmeh <adeshmeh@ca.ibm.com>

cli/compose/interpolation/interpolation.go

@@ -39,7 +39,7 @@ func interpolateSectionItem(
 		interpolatedValue, err := recursiveInterpolate(value, mapping)
 		if err != nil {
 			return nil, fmt.Errorf(
-				"Invalid interpolation format for %#v option in %s %#v: %#v",
+				"Invalid interpolation format for %#v option in %s %#v: %#v. You may need to escape any $ with another $.",
 				key, section, name, err.Template,
 			)
 		}

cli/compose/interpolation/interpolation_test.go

@@ -55,5 +55,5 @@ func TestInvalidInterpolation(t *testing.T) {
 		},
 	}
 	_, err := Interpolate(services, "service", defaultMapping)
-	assert.EqualError(t, err, `Invalid interpolation format for "image" option in service "servicea": "${"`)
+	assert.EqualError(t, err, `Invalid interpolation format for "image" option in service "servicea": "${". You may need to escape any $ with another $.`)
 }