diff --git a/Dockerfile b/Dockerfile
index 4d4b649..0598a01 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -16,6 +16,12 @@ RUN CGO_ENABLED=0 GOOS=${TARGETOS} GOARCH=${TARGETARCH} \
     go build -ldflags="-w -s -X main.version=${VERSION}" -o /terdut ./cmd/terdut
 
 FROM scratch
+# scratch has no trust store, and a Go binary on it fails every HTTPS call with
+# "x509: certificate signed by unknown authority". Nothing needed one until single
+# sign-on: discovery and the token exchange are HTTPS calls to the identity provider.
+# The bundle is the builder's, copied by name so a missing file fails the build
+# rather than shipping an image that cannot sign anybody in.
+COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/ca-certificates.crt
 COPY --from=builder /terdut /terdut
 EXPOSE 8080
 ENTRYPOINT ["/terdut"]