Add wp-image-[ID] class to img tag so WP adds srcset

The AB Container block background image did not have a srcset.
Adding the class wp-image-[ID] to the img tag triggers WP
to add a srcset to the image. For more info see:
WordPress/gutenberg#8593

src/blocks/block-container/index.js

@@ -229,7 +229,7 @@ registerBlockType( 'atomic-blocks/ab-container', {
 						<div className="ab-container-image-wrap">
 							<img
 								className={ classnames(
-									'ab-container-image',
+									'ab-container-image wp-image-' . containerImgID,
 									dimRatioToClass( containerDimRatio ),
 									{
 										'has-background-dim': 0 !== containerDimRatio