adds platform environment variables related content from the spec (#771)

* adds platform env var related content from the spec

Signed-off-by: Hanan Younes <[email protected]>

---------

Signed-off-by: Hanan Younes <[email protected]>

content/docs/for-platform-operators/how-to/build-inputs/build-env.md

@@ -0,0 +1,34 @@
++++
+title="Specify the build time environment variables"
+weight=4
++++
+
+`Environment variables` are used to configure buildpack behavior. They may be specified by:
+
+* The platform operator (this page)
+* The end user (see [Customize buildpack behavior with build-time environment variables][end user])
+* Other buildpacks (see [Specify the environment][env])
+
+<!--more-->
+
+When more than one entity specifies the same `environment variable`, the order of precedence is as shown above, with the platform operator having ultimate say over what the final value of the variable will be.
+
+The platform operator specifies `environment variables` in a manner that is very similar to buildpacks (see XXX), but with a few differences. Namely:
+
+* The directory for environment variable settings `/cnb/build-config`
+* When no suffix is provided, the modification behavior is `default`
+For more information, consult the [Platform Specification](https://github.com/buildpacks/spec/blob/main/platform.md).
+
+### Example
+
+Platform operators can make choices that "override" or provide defaults for application authors.  In the following configuration the platform operator overrides the value of `CGO_ENABLED` for all application authors.  The value of `PIP_VERBOSE` is set by default and can be overridden by buildpack authors or application authors.  Any value for the environment variable `CLASSPATH` is prepended with the values provided by the platform operator.
+
+```bash
+$ tree /cnb/buildconfig/env
+├── CGO_ENABLED.override
+├── PIP_VERBOSE
+└── CLASSPATH.prepend
+```
+
+[env]: https://buildpacks.io/docs/for-buildpack-authors/how-to/write-buildpacks/specify-env/
+[end user]: https://buildpacks.io/docs/for-app-developers/how-to/build-inputs/configure-build-time-environment/