Merge 1fe56af526 into 1d2bff5959

Add more documentation for the --build-arg parameter of the executor.
Describe that it's possible to lookup build argument values from the
environment and that this might be useful if you don't want to fiddle
around with shell escaping.

README.md

@@ -806,6 +806,18 @@ export IFS=''
 /kaniko/executor --build-arg "MY_VAR='value with spaces'" ...
 ```
 
+It is also possible to lookup the build argument value from the environment
+variables which is especially useful when you already have the build argument
+value in the environment variables and don't want to fiddle around with shell escaping.
+To use an environment variable as build argument value, you just need to
+use the `--build-arg` parameter just with the name of the argument:
+
+```bash
+/kaniko/executor --build-arg "MY_VAR" ...
+```
+In this example, Kaniko will then set the build argument `MY_VAR` to the value of the environment
+variable `MY_VAR`.
+
 #### Flag `--cache`
 
 Set this flag as `--cache=true` to opt into caching with kaniko.