fix(signup): copy manifest.toml so Docker builds use the lockfile

Commit 40011e2 added manifest.toml to pin transitive deps, but the
Dockerfile only copies gleam.toml — so every Docker build still
re-resolves from scratch and can pick up incompatible versions within
the allowed ranges, defeating the lockfile's purpose.

signup/Dockerfile

@@ -1,6 +1,6 @@
 FROM ghcr.io/gleam-lang/gleam:v1.16.0-erlang-alpine AS builder
 WORKDIR /build
-COPY gleam.toml ./
+COPY gleam.toml manifest.toml ./
 RUN gleam deps download
 COPY src ./src
 RUN gleam export erlang-shipment