Call Sim: Speed up docker builds

.dockerignore

@@ -1,5 +1,7 @@
 out/
 out-arm/
+target/
+!target/release/call_sim-cli
 cscope.*
 .DS_Store
 .dir-locals.el

call_sim/docker/signaling_server/Dockerfile

@@ -1,5 +1,3 @@
-# syntax=docker/dockerfile:1.7-labs
-
 #
 # Copyright 2023 Signal Messenger, LLC
 # SPDX-License-Identifier: AGPL-3.0-only
@@ -13,37 +11,30 @@ RUN apt-get update \
     && apt-get install -y --no-install-recommends --no-install-suggests curl build-essential ca-certificates protobuf-compiler \
     && update-ca-certificates
 
-SHELL [ "/bin/bash", "-c"]
-
 # Install Rust.
 RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
 ENV PATH="/root/.cargo/bin:${PATH}"
 
-# Since protobuf is a workspace package, we have to stub the entire workspace
 WORKDIR /usr/src/ringrtc
-COPY --parents ./**/Cargo.toml ./**/Cargo.lock ./**/main.rs ./**/lib.rs ./**/bin/*.rs ./
-RUN shopt -s globstar; shopt -s nullglob; \
-    for filename in ./**/main.rs; do \
-        truncate -s 0 "$filename"; \
-    done; \
-    for filename in ./**/lib.rs; do \
-        truncate -s 0 "$filename"; \
-    done; \
-    for filename in ./**/bin/*.rs; do \
-        truncate -s 0 "$filename"; \
-    done
-# stub the signaling server, but remove the actual code first
-RUN cd call_sim/docker && \
-    rm -rf signaling_server && \
-    cargo new signaling_server
-COPY call_sim/docker/signaling_server/Cargo.* ./call_sim/docker/signaling_server/
-RUN cd call_sim/docker/signaling_server && cargo build --release
 
-# Copy the source and build the project normally.
+# Minimal workspace root so cargo can resolve protobuf's inherited fields.
+RUN cat <<'EOF' > Cargo.toml
+[workspace]
+resolver = "2"
+members = ["protobuf"]
+
+[workspace.package]
+version = "0.0.0"
+authors = []
+EOF
+
+# Copy only the crates signaling_server actually depends on.
 COPY protobuf protobuf
-RUN cd protobuf && cargo build --release
 
+# Copy the signaling_server project.
 COPY call_sim/docker/signaling_server call_sim/docker/signaling_server
+
+# Build.
 RUN cd call_sim/docker/signaling_server && cargo build --release
 
 FROM ubuntu:22.04 AS run-stage