chore(tests): use multi-stage Dockerfile to skip source copy for integration runner (#25646)

Author: pront

.github/workflows/ci-integration-review.yml

@@ -164,6 +164,7 @@ jobs:
         if: steps.run_condition.outputs.should_run == 'true'
 
       - uses: ./.github/actions/setup
+        if: steps.run_condition.outputs.should_run == 'true'
         with:
           vdev: true
           datadog-ci: true
@@ -222,6 +223,7 @@ jobs:
         if: steps.run_condition.outputs.should_run == 'true'
 
       - uses: ./.github/actions/setup
+        if: steps.run_condition.outputs.should_run == 'true'
         with:
           vdev: true
           datadog-ci: true

tests/e2e/Dockerfile

@@ -2,7 +2,7 @@ ARG RUST_VERSION=1
 ARG FEATURES
 ARG BUILD=false
 
-FROM docker.io/rust:${RUST_VERSION}-slim-trixie
+FROM docker.io/rust:${RUST_VERSION}-slim-trixie AS base
 
 COPY scripts/environment/install-debian-build-deps.sh /
 RUN bash /install-debian-build-deps.sh
@@ -19,15 +19,19 @@ COPY rust-toolchain.toml .
 RUN bash /prepare.sh --modules=cargo-nextest,cargo-llvm-cov
 RUN bash /install-protoc.sh
 
+FROM base AS build-false
+
+FROM base AS build-true
 COPY . .
 ARG FEATURES
-ARG BUILD
-
 RUN --mount=type=cache,target=/vector/target \
     --mount=type=cache,target=/usr/local/cargo/registry \
     --mount=type=cache,target=/usr/local/cargo/git \
-    if [ "$BUILD" = "true" ]; then \
-        /usr/bin/mold -run cargo build --tests --lib --bin vector \
-        --no-default-features --features $FEATURES && \
-        cp target/debug/vector /usr/bin/vector; \
-    fi
+    /usr/bin/mold -run cargo build --tests --lib --bin vector \
+    --no-default-features --features $FEATURES && \
+    cp target/debug/vector /usr/bin/vector
+
+# BuildKit conditional stage selection: selects build-false (lean, no source) or
+# build-true (full source + compiled binary) based on the BUILD arg.
+# https://www.docker.com/blog/advanced-dockerfiles-faster-builds-and-smaller-images-using-buildkit-and-multistage-builds/
+FROM build-${BUILD}