Clarify label and layer guidance (#4992)

engine/userguide/eng-image/dockerfile_best-practices.md

@@ -99,9 +99,19 @@ If containers depend on each other, you can use [Docker container networks](http
 
 ### Minimize the number of layers
 
-You need to find the balance between readability (and thus long-term
-maintainability) of the `Dockerfile` and minimizing the number of layers it
-uses. Be strategic and cautious about the number of layers you use.
+Prior to Docker 17.05, and even more, prior to Docker 1.10, it was important
+to minimize the number of layers in your image. The following improvements have
+mitigated this need:
+
+- In Docker 1.10 and higher, only `RUN`, `COPY`, and `ADD` instructions create
+  layers. Other instructions create temporary intermediate images, and no longer
+  directly increase the size of the build.
+
+- Docker 17.05 and higher add support for
+  [multi-stage builds](multistage-build.md), which allow you to copy only the
+  artifacts you need into the final image. This allows you to include tools and
+  debug information in your intermediate build stages without increasing the
+  size of the final image.
 
 ### Sort multi-line arguments
 
@@ -192,10 +202,10 @@ LABEL com.example.release-date="2015-02-12"
 LABEL com.example.version.is-production=""
 ```
 
-An image can have more than one label. To specify multiple labels, Docker
-recommends combining labels into a single `LABEL` instruction where possible.
-Each `LABEL` instruction produces a new layer which can result in an inefficient
-image if you use many labels. This example results in a single image layer.
+An image can have more than one label. Prior to Docker 1.10, it was recommended
+to combine all labels into a single `LABEL` instruction, to prevent extra layers
+from being created. This is no longer necessary, but combining labels is still
+supported.
 
 ```conf
 # Set multiple labels on one line