fix: copy migrations/ directory into Docker build context

The `sqlx::migrate!("./migrations")` macro requires the `migrations`
directory at compile time. The `Dockerfile` was missing a `COPY` for it.

Author: claymcleod

Dockerfile

@@ -16,6 +16,7 @@ COPY ./src ./src
 COPY ./benches ./benches
 COPY ./crates ./crates
 COPY ./tests ./tests
+COPY ./migrations ./migrations
 
 # Build the release version of Sprocket
 RUN cargo build --release