Give the image CA certificates so fetch works over HTTPS (closes #131)

The final stage is scratch, which has no CA certificates, so fetch from
an HTTPS URL failed to verify any server. Copy the CA bundle from the
pinned builder image into the final stage.

Model: opus-5-5

Dockerfile

@@ -75,5 +75,7 @@ RUN version="${VERSION:-$(git describe --tags --always)}"; \
 RUN ldd /mfer 2>&1 | grep -q 'not a dynamic executable'
 
 FROM scratch
+# scratch has no CA certificates; fetch needs them to verify HTTPS servers.
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
 COPY --from=builder /mfer /mfer
 ENTRYPOINT ["/mfer"]