Support metaarg expansion

[jedinym]

This PR adds support for metaarg expansion if an argument is nested.

Example Dockerfile:

ARG ALPINE_MINOR=10
ARG ALPINE_TAG=3.${ALPINE_MINOR}
ARG APP_BASE=scratch

FROM alpine:${ALPINE_TAG} AS build
# ...

FROM build
# ...

FROM ${APP_BASE}
# ...

The master version does not support argument nesting like that, resulting in unexpanded variables. Comparing the master version with this branch shows the difference:

$ dockerfile-json examples/Dockerfile.3

--- actual      2025-08-27 10:06:40.138591062 +0200
+++ expected    2025-08-27 10:05:48.028701625 +0200
@@ -10,7 +10,7 @@
       "Key": "ALPINE_TAG",
       "DefaultValue": "3.${ALPINE_MINOR}",
       "ProvidedValue": null,
-      "Value": "3.${ALPINE_MINOR}"
+      "Value": "3.10"
     },
     {
       "Key": "APP_BASE",
@@ -22,7 +22,8 @@
   "Stages": [
     {
       "Name": "build",
-      "BaseName": "alpine:3.${ALPINE_MINOR}",
+      "OrigCmd": "FROM",
+      "BaseName": "alpine:3.10",
       "Platform": "",
       "Comment": "",
       "SourceCode": "FROM alpine:${ALPINE_TAG} AS build",
@@ -40,12 +41,13 @@
       ],
       "As": "build",
       "From": {
-        "Image": "alpine:3.${ALPINE_MINOR}"
+        "Image": "alpine:3.10"
       },
       "Commands": null
     },
     {
       "Name": "",
+      "OrigCmd": "FROM",
       "BaseName": "build",
       "Platform": "",
       "Comment": "",
@@ -72,6 +74,7 @@
     },
     {
       "Name": "",
+      "OrigCmd": "FROM",
       "BaseName": "scratch",
       "Platform": "",
       "Comment": "",

Attaching full outputs of master branch and this branch below.
actual.json
expected.json

[mkosiarc]

@sgreben could you please take a look here?

[jedinym]

@sgreben have you had the chance to take a look at this?