fix(docker): remove unneeded openssl deps, verify sqlx offline cache path

All crypto uses pure-Rust backends: jsonwebtoken with rust_crypto feature,
sqlx with runtime-tokio-rustls, reqwest with rustls. Neither libssl-dev
(builder) nor libssl3 (runtime) are required. pkg-config is also removed
as no build.rs in the dep tree invokes it. EXPOSE updated to reflect the
SERVER_PORT env var with an 8080 fallback.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -6,10 +6,6 @@ FROM rust:slim AS builder
 
 WORKDIR /app
 
-RUN apt-get update \
-    && apt-get install -y pkg-config libssl-dev \
-    && rm -rf /var/lib/apt/lists/*
-
 COPY . .
 
 # Tell sqlx to use the cached query metadata instead of a live database.
@@ -22,11 +18,11 @@ RUN cargo build --release -p solitaire_server
 FROM debian:bookworm-slim
 
 RUN apt-get update \
-    && apt-get install -y libssl3 ca-certificates \
+    && apt-get install -y ca-certificates \
     && rm -rf /var/lib/apt/lists/*
 
 COPY --from=builder /app/target/release/solitaire_server /usr/local/bin/solitaire_server
 
-EXPOSE 8080
+EXPOSE ${SERVER_PORT:-8080}
 
 ENTRYPOINT ["/usr/local/bin/solitaire_server"]