fix: give the scratch server image a /tmp for the vulnerability database

The runtime stage is FROM scratch, which has no /tmp, so vulnsched died at
startup with "temp dir: stat /tmp: no such file or directory" and no scan
ever ran. Nothing in the server wrote to a temporary directory before the
trivy-db puller, which is why this only appeared now.

scratch cannot mkdir its own, so the directory is staged in the builder at
1777 and copied in. Also corrects CLAUDE.md, which described this image as
Alpine; the time/tzdata import it justifies is if anything more load-bearing
on scratch.

server/Dockerfile

@@ -18,10 +18,19 @@ ARG VERSION=dev
 RUN cd server && CGO_ENABLED=0 GOOS=linux go build \
       -ldflags="-s -w -X main.Version=${VERSION}" -o /vantage-server ./cmd
 
+# Staged so the scratch image below can have a /tmp. It cannot mkdir one
+# itself — scratch has no shell — and os.MkdirTemp fails outright without it.
+RUN mkdir -p /staging/tmp && chmod 1777 /staging/tmp
+
 # Runtime stage
 FROM scratch
 
 COPY --from=builder /etc/ssl/certs/ca-certificates.crt /etc/ssl/certs/
+
+# vulndb unpacks the ~50MB trivy-db here. Without it the scheduler stops at
+# "temp dir: stat /tmp: no such file or directory" and no scanning happens,
+# while everything else in the process runs perfectly well.
+COPY --from=builder /staging/tmp /tmp
 COPY --from=builder /vantage-server /vantage-server
 
 COPY default_steps/ /opt/default-steps/