fix: Simplify Dockerfile to use sqlx offline mode

- Remove complex build caching
- Set SQLX_OFFLINE=true to skip compile-time query verification
- Queries still validated at runtime
- Eliminates need for DATABASE_URL during Docker build

docker/Dockerfile.api

@@ -6,16 +6,15 @@ RUN apt-get update && apt-get install -y pkg-config libssl-dev && rm -rf /var/li
 
 WORKDIR /build
 
-# Cache dependencies — copy manifests first
-COPY Cargo.toml ./
-COPY Cargo.lock* ./
-RUN mkdir src && echo "fn main() {}" > src/main.rs
-RUN cargo build --release 2>/dev/null || true
-
-# Now copy actual source and build
+# Copy everything
+COPY Cargo.toml Cargo.lock* ./
 COPY src/ src/
 COPY migrations/ migrations/
-RUN touch src/main.rs && cargo build --release
+
+# Build with sqlx offline mode to skip compile-time query verification
+# Runtime validation still occurs - just no compile-time type checking
+ENV SQLX_OFFLINE=true
+RUN cargo build --release
 
 # Stage 2: Runtime image
 FROM debian:bookworm-slim
@@ -25,7 +24,7 @@ RUN apt-get update && apt-get install -y ca-certificates && rm -rf /var/lib/apt/
 WORKDIR /app
 
 COPY --from=builder /build/target/release/corrosion-api .
-COPY migrations/ migrations/
+COPY --from=builder /build/migrations/ migrations/
 
 ENV RUST_LOG=corrosion_api=info,tower_http=info