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.

build.gradle

@@ -78,7 +78,7 @@ springBoot {
 
 allprojects {
     group = 'stirling.software'
-    version = '2.13.1'
+    version = '2.13.2'
 
     configurations.configureEach {
         exclude group: "org.springframework.boot", module: "spring-boot-starter-tomcat"