builder: rephrase ENV section, remove examples for ENV key value without '='

The `ENV key value` form can be ambiguous, for example, the following defines
a single env-variable (`ONE`) with value `"TWO= THREE=world"`:

    ENV ONE TWO= THREE=world

While we cannot deprecate/remove that syntax (as it would break existing
Dockerfiles), we should reduce exposure of the format in our examples.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/reference/commandline/build.md

@@ -742,7 +742,7 @@ FROM busybox
 RUN echo hello > /hello
 RUN echo world >> /hello
 RUN touch remove_me /remove_me
-ENV HELLO world
+ENV HELLO=world
 RUN rm /remove_me
 ```
 

docs/reference/commandline/commit.md

@@ -73,7 +73,7 @@ $ docker inspect -f "{{ .Config.Env }}" c3f279d17e0a
 
 [HOME=/ PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin]
 
-$ docker commit --change "ENV DEBUG true" c3f279d17e0a  svendowideit/testimage:version3
+$ docker commit --change "ENV DEBUG=true" c3f279d17e0a  svendowideit/testimage:version3
 
 f5283438590d
 

docs/reference/commandline/import.md

@@ -72,7 +72,7 @@ $ sudo tar -c . | docker import - exampleimagedir
 ### Import from a local directory with new configurations
 
 ```bash
-$ sudo tar -c . | docker import --change "ENV DEBUG true" - exampleimagedir
+$ sudo tar -c . | docker import --change "ENV DEBUG=true" - exampleimagedir
 ```
 
 Note the `sudo` in this example – you must preserve