Docs for new [[build.env]] table in builder.toml

content/docs/reference/config/builder-config.md

@@ -95,6 +95,23 @@ A [builder][builder] configuration schema is as follows:
     > at the time of `pack`'s release. In other words, for a particular version of `pack`, this default
     > will not change despite new lifecycle versions being released.
 
+- #### `build.env` _(optional)_
+  The [[[build.env]]](https://github.com/buildpacks/spec/blob/main/buildpack.md#environment-variable-modification-rules) is used to specify [operator-defined](https://github.com/buildpacks/spec/blob/main/platform.md#operator-defined-variables) build-time environment variables for buildpacks. Set `CNB_BUILD_CONFIG_DIR` in pack's environment to override the default directory (`/cnb/build-config/env`) where environment variable files are stored within the builder.
+
+  - **`name`** _(string, required)_\
+    The name/key of the environment variable.
+
+  - **`value`** _(string, required)_\
+    The value of the environment variable.
+
+  - **`suffix`** _(string, optional)_\
+    The type of action used to modify the environment variable when end-users or buildpacks define the same name/key, one of [`default`](https://github.com/buildpacks/spec/blob/main/buildpack.md#default), [`override`](https://github.com/buildpacks/spec/blob/main/buildpack.md#override), [`append`](https://github.com/buildpacks/spec/blob/main/buildpack.md#append), or [`prepend`](https://github.com/buildpacks/spec/blob/main/buildpack.md#prepend). Defaults to `default` if this field is omitted. Operator-defined environment variables take precedence over end-user or buildpack-defined environment variables.
+
+  - **`delim`** _(string, optional)_\
+    The delimiter used to concatenate two or more values for the given `name`.
+
+  > The `delim` is required when `suffix` is one of `append` or `prepend`. 
+
 ### Supported archives
 
 Currently, when specifying a URI to a buildpack or lifecycle, only `tar` or `tgz` archive types are supported.