Build a static binary so the scratch image runs (closes #126)

The final stage is scratch, which has no C library, but the builder
compiled mfer with cgo on (the golang image's default). The binary
imports net (mfer's own HTTP code does, and so does google/uuid), so
with cgo on it came out dynamically linked and the image could not
start. The image's go build now sets CGO_ENABLED=0; nothing in mfer
needs cgo. A new builder step runs ldd on the binary and fails the
build unless it reports a static executable.

Model: opus-5-5

Dockerfile

@@ -67,7 +67,10 @@ RUN version="${VERSION:-$(git describe --tags --always)}"; \
         exit 1; \
     fi; \
     cd cmd/mfer && \
-    go build -tags urfave_cli_no_docs -ldflags "-X main.Gitrev=$version" -o /mfer .
+    CGO_ENABLED=0 go build -tags urfave_cli_no_docs -ldflags "-X main.Gitrev=$version" -o /mfer .
+
+# Fail unless /mfer is statically linked: scratch has no C library to run it.
+RUN ldd /mfer 2>&1 | grep -q 'not a dynamic executable'
 
 FROM scratch
 COPY --from=builder /mfer /mfer