Use @ digest for BASE_VERSION in Dockerfiles

Replace the colon with '@' in BASE_VERSION ARG values to use the proper image digest syntax (tag@sha256:...) across docker/backend/Dockerfile, docker/embedded/Dockerfile, and docker/embedded/Dockerfile.fat. This makes the BASE_IMAGE ARG reference a canonical digest for more reliable and reproducible base image resolution.

docker/backend/Dockerfile

@@ -1,6 +1,6 @@
 # Stirling-PDF backend-only image — JAR built with -PbuildWithFrontend=false, UI ships separately.
 
-ARG BASE_VERSION=1.0.2:sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
+ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application (backend only, no frontend)

docker/embedded/Dockerfile

@@ -1,7 +1,7 @@
 # Stirling-PDF - Full version (embedded frontend)
 # Uses pre-built base image for fast builds
 
-ARG BASE_VERSION=1.0.2:sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
+ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application and frontend

docker/embedded/Dockerfile.fat

@@ -2,7 +2,7 @@
 # Extra fonts for air-gapped environments
 # Uses pre-built base image for fast builds
 
-ARG BASE_VERSION=1.0.2:sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
+ARG BASE_VERSION=1.0.2@sha256:c7698687f486707ddef9e0298587ca8b44c4e96185e1bdb0c3d119eb2bf9a82e
 ARG BASE_IMAGE=stirlingtools/stirling-pdf-base:${BASE_VERSION}
 
 # Stage 1: Build the Java application and frontend