make the cache miss clear
@@ -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]
