Remove ffmpeg from published Docker images (#6791)

## Summary

Published Docker images (`stirling-pdf:latest`, `:2.13.1`) still shipped
the full `ffmpeg` package even though it was disabled in source back in
#6053.

**Root cause:** `push-docker.yml` passed a hardcoded
`BASE_VERSION=1.0.0` build-arg for the regular image, overriding the
Dockerfile's `ARG BASE_VERSION=1.0.2` default. Base `1.0.0` is the
original base that still does the explicit `ffmpeg` apt install, so the
published image never picked up the removal.

.github/workflows/push-docker.yml

@@ -155,9 +155,9 @@ jobs:
           cache-to: type=gha,mode=max,scope=stirling-pdf-latest
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}
+          # No BASE_VERSION pin: inherit the Dockerfile ARG default (single source of truth).
           build-args: |
             VERSION_TAG=${{ steps.versionNumber.outputs.versionNumber }}
-            BASE_VERSION=1.0.0
           platforms: linux/amd64,linux/arm64/v8
           provenance: true
           sbom: true