fix: docker build out-of-disk issue - aggressive cleanup

- .dockerignore: exclude build/cache/docs to reduce build context (59 lines)
- Dockerfile: add aggressive cleanup (cache, registry, git) after cargo build
- Dockerfile: use --locked flag for reproducible builds
- build.yaml: add pre-build disk cleanup (docker + cargo cache)
- build.yaml: clean cargo before Docker build to free space
- build.yaml: show disk usage before/after for debugging

Fixes 'No space left on device' error during Docker build in runner

Dockerfile

@@ -5,14 +5,23 @@ FROM rust:1-bookworm as builder
 
 WORKDIR /build
 
+# Build settings
+ENV SQLX_OFFLINE=true
+
 # Copy source
 COPY . .
 
-# Build the mem binary (offline sqlx - uses .sqlx/ cache)
-ENV SQLX_OFFLINE=true
-RUN cargo build --release -p mem-cli && \
+# Build release binary with space-efficient cleanup
+RUN cargo build --release -p mem-cli --locked && \
     strip target/release/mem && \
-    rm -rf target/release/deps target/release/build target/release/incremental target/release/.fingerprint
+    # Aggressive cleanup to free disk space
+    rm -rf target/release/deps && \
+    rm -rf target/release/build && \
+    rm -rf target/release/incremental && \
+    rm -rf target/release/.fingerprint && \
+    rm -rf .cargo/registry/cache && \
+    rm -rf .cargo/registry/index && \
+    rm -rf .cargo/git
 
 # Stage 2: Runtime
 FROM debian:bookworm-slim