docs: rework bake "Configuring builds" page

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

docs/guides/bake/configuring-build.md

@@ -3,6 +3,14 @@ title: "Configuring builds"
 keywords: build, buildx, bake, buildkit, hcl, json
 ---
 
+Bake supports loading build definition from files, but sometimes you need even
+more flexibility to configure this definition.
+
+For this use case, you can define variables inside the bake files that can be
+set by the user with environment variables or by [attribute definitions](#global-scope-attributes)
+in other bake files. If you wish to change a specific value for a single
+invocation you can use the `--set` flag [from the command line](#from-command-line).
+
 ## Global scope attributes
 
 You can define global scope attributes in HCL/JSON and use them for code reuse
@@ -85,7 +93,8 @@ $ docker buildx bake -f docker-bake.hcl -f env.hcl --print app
 
 ## From command line
 
-You can also override target configurations from the command line with [`--set` flag](https://docs.docker.com/engine/reference/commandline/buildx_bake/#set):
+You can also override target configurations from the command line with the
+[`--set` flag](https://docs.docker.com/engine/reference/commandline/buildx_bake/#set):
 
 ```hcl
 # docker-bake.hcl
@@ -148,3 +157,63 @@ Complete list of overridable fields:
 * `ssh`
 * `tags`
 * `target`
+
+## Using variables in variables across files
+
+When multiple files are specified, one file can use variables defined in
+another file.
+
+```hcl
+# docker-bake1.hcl
+variable "FOO" {
+  default = upper("${BASE}def")
+}
+
+variable "BAR" {
+  default = "-${FOO}-"
+}
+
+target "app" {
+  args = {
+    v1 = "pre-${BAR}"
+  }
+}
+```
+
+```hcl
+# docker-bake2.hcl
+variable "BASE" {
+  default = "abc"
+}
+
+target "app" {
+  args = {
+    v2 = "${FOO}-post"
+  }
+}
+```
+
+```console
+$ docker buildx bake -f docker-bake1.hcl -f docker-bake2.hcl --print app
+```
+```json
+{
+  "group": {
+    "default": {
+      "targets": [
+        "app"
+      ]
+    }
+  },
+  "target": {
+    "app": {
+      "context": ".",
+      "dockerfile": "Dockerfile",
+      "args": {
+        "v1": "pre--ABCDEF-",
+        "v2": "ABCDEF-post"
+      }
+    }
+  }
+}
+```