@@ -1174,8 +1174,10 @@ To use these, simply pass them on the command line using the `--build-arg

 `ARG` variables are not persisted into the built image as `ENV` variables are.

 However, `ARG` variables do impact the build cache in similar ways. If a

 Dockerfile defines an `ARG` variable whose value is different from a previous

-build, then a "cache miss" occurs upon its first usage, not its declaration.

-For example, consider this Dockerfile:

+build, then a "cache miss" occurs upon first use of the `ARG` variable. The

+declaration of the `ARG` variable does not count as a use.

+

+For example, consider these two Dockerfile:

 ```

 1 FROM ubuntu

@@ -1183,12 +1185,17 @@ For example, consider this Dockerfile:

 3 RUN echo $CONT_IMG_VER

 ```

-If you specify `--build-arg CONT_IMG_VER=<value>` on the command line the

-specification on line 2 does not cause a cache miss; line 3 does cause a cache

-miss. The definition on line 2 has no impact on the resulting image. The `RUN`

-on line 3 executes a command and in doing so defines a set of environment

-variables, including `CONT_IMG_VER`. At that point, the `ARG` variable may

-impact the resulting image, so a cache miss occurs.

+```

+1 FROM ubuntu

+2 ARG CONT_IMG_VER

+3 RUN echo hello

+```

+

+If you specify `--build-arg CONT_IMG_VER=<value>` on the command line, in both

+cases, the specification on line 2 does not cause a cache miss; line 3 does

+cause a cache miss.`ARG CONT_IMG_VER` causes the RUN line to be identified

+as the same as running `CONT_IMG_VER=<value>` echo hello, so if the `<value>`

+changes, we get a cache miss.

 Consider another example under the same command line:

@@ -1203,6 +1210,20 @@ the variable's value in the `ENV` references the `ARG` variable and that

 variable is changed through the command line. In this example, the `ENV`

 command causes the image to include the value.

+If an `ENV` instruction overrides an `ARG` instruction of the same name, like

+this Dockerfile:

+

+```

+1 FROM ubuntu

+2 ARG CONT_IMG_VER

+3 ENV CONT_IMG_VER hello

+4 RUN echo $CONT_IMG_VER

+```

+

+Line 3 does not cause a cache miss because the value of `CONT_IMG_VER` is a

+constant (`hello`). As a result, the environment variables and values used on

+the `RUN` (line 4) doesn't change between builds.

+

 ## ONBUILD

     ONBUILD [INSTRUCTION]