lint: add doc for UndefinedArgInFrom

Signed-off-by: David Karlsson <[email protected]>

Author: dvdksn

frontend/dockerfile/docs/rules/undefined-arg-in-from.md

@@ -8,19 +8,47 @@ aliases:
 ## Output
 
 ```text
+FROM argument 'VARIANT' is not declared
 ```
 
 ## Description
 
+This rule warns for cases where you're consuming an undefined build argument in
+`FROM` instructions.
+
+Interpolating build arguments in `FROM` instructions can be a good way to add
+flexibility to your build, and lets you pass arguments that overriding the base
+image of a stage. For example, you might use a build argument to specify the
+image tag:
+
+```dockerfile
+ARG ALPINE_VERSION=3.20
+
+FROM alpine:${ALPINE_VERSION}
+```
+
+This makes it possible to run the build with a different `alpine` version by
+specifying a build argument:
+
+```console
+$ docker buildx build --build-arg ALPINE_VERSION=edge .
+```
+
+This check also tries to detect and warn when a `FROM` instruction reference
+miss-spelled built-in build arguments, like `BUILDPLATFORM`.
+
 ## Examples
 
-❌ Bad
+❌ Bad: the `VARIANT` build argument is undefined.
 
 ```dockerfile
+FROM node:22${VARIANT} AS jsbuilder
 ```
 
-✅ Good
+✅ Good: the `VARIANT` build argument is defined.
 
 ```dockerfile
+ARG VARIANT="-alpine3.20"
+FROM node:22${VARIANT} AS jsbuilder
 ```
 

frontend/dockerfile/linter/docs/UndefinedArgInFrom.md

@@ -1,18 +1,46 @@
 ## Output
 
 ```text
+FROM argument 'VARIANT' is not declared
 ```
 
 ## Description
 
+This rule warns for cases where you're consuming an undefined build argument in
+`FROM` instructions.
+
+Interpolating build arguments in `FROM` instructions can be a good way to add
+flexibility to your build, and lets you pass arguments that overriding the base
+image of a stage. For example, you might use a build argument to specify the
+image tag:
+
+```dockerfile
+ARG ALPINE_VERSION=3.20
+
+FROM alpine:${ALPINE_VERSION}
+```
+
+This makes it possible to run the build with a different `alpine` version by
+specifying a build argument:
+
+```console
+$ docker buildx build --build-arg ALPINE_VERSION=edge .
+```
+
+This check also tries to detect and warn when a `FROM` instruction reference
+miss-spelled built-in build arguments, like `BUILDPLATFORM`.
+
 ## Examples
 
-❌ Bad
+❌ Bad: the `VARIANT` build argument is undefined.
 
 ```dockerfile
+FROM node:22${VARIANT} AS jsbuilder
 ```
 
-✅ Good
+✅ Good: the `VARIANT` build argument is defined.
 
 ```dockerfile
+ARG VARIANT="-alpine3.20"
+FROM node:22${VARIANT} AS jsbuilder
 ```