COMP: Remove docker build warning

- LegacyKeyValueFormat: "ENV key=value" should be used
  instead of legacy "ENV key value" format (line 42)

"""
The new format for declaring environment variables in Dockerfiles is ENV
key=value. This format, where the key and value are separated by an
equals sign (=), is the recommended approach. The older format, ENV key
value (with a space as a separator), is deprecated.
""" -- google search

README.md

@@ -569,7 +569,7 @@ An example Dockerfile would be:
 ```
 FROM dockcross/linux-armv7
 
-ENV DEFAULT_DOCKCROSS_IMAGE my_cool_image
+ENV DEFAULT_DOCKCROSS_IMAGE=my_cool_image
 RUN apt-get install -y nano
 ```