fix(docker): set SOPS_AGE_KEY as ENV from ARG for build stage

ARG values are not automatically available as environment variables
in RUN commands. Convert ARG to ENV so sops can access the key.

Author: aster-void

Dockerfile

@@ -22,6 +22,7 @@ RUN bun run prepare
 
 # Build with sops secrets
 ARG SOPS_AGE_KEY
+ENV SOPS_AGE_KEY=${SOPS_AGE_KEY}
 RUN sops exec-env secrets.prod.yaml 'bun run build'
 
 FROM base AS executor