diff --git a/.gitignore b/.gitignore
index ce9b98e..d3bbb5e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -33,6 +33,11 @@ __pycache__/
 *.egg-info/
 *.egg
 
+# Per-run build contexts created by testing/ci-local.sh. Its teardown normally
+# removes them, but the CI worker's SIGKILL runs no trap, so one can survive a
+# preempted run; ci-cleanup.sh sweeps the survivors.
+/testing/docker-*/
+
 # Runtime artifacts from running fips in-tree during local testing.
 # Root-anchored so legitimately-tracked fips.yaml under packaging/ and
 # examples/ stays included.
diff --git a/testing/acl-allowlist/docker-compose.yml b/testing/acl-allowlist/docker-compose.yml
index 9bcb02e..a20428a 100644
--- a/testing/acl-allowlist/docker-compose.yml
+++ b/testing/acl-allowlist/docker-compose.yml
@@ -19,7 +19,11 @@ networks:
 
 x-fips-common: &fips-common
   build:
-    context: ../docker
+    # The harness scopes its build context per run and passes it here; the
+    # shared directory is the hand-run default. Compose resolves a relative
+    # value against THIS file's directory, so the harness must export an
+    # absolute path.
+    context: ${FIPS_BUILD_CONTEXT:-../docker}
   image: ${FIPS_TEST_IMAGE:-fips-test:latest}
   entrypoint: ["/usr/local/bin/entrypoint.sh"]
   cap_add:
diff --git a/testing/chaos/scripts/chaos.sh b/testing/chaos/scripts/chaos.sh
index 0ffdbb9..2a22d2b 100755
--- a/testing/chaos/scripts/chaos.sh
+++ b/testing/chaos/scripts/chaos.sh
@@ -125,7 +125,11 @@ if ! docker info &> /dev/null; then
     exit 1
 fi
 
-DOCKER_DIR="$CHAOS_DIR/../docker"
+# A harness that scopes its build context per run passes it in
+# FIPS_BUILD_CONTEXT and stops writing to the shared directory, so checking the
+# shared one would either fail on a clean checkout or, worse, pass while
+# reading a stale binary that is not the one under test.
+DOCKER_DIR="${FIPS_BUILD_CONTEXT:-$CHAOS_DIR/../docker}"
 if [ ! -f "$DOCKER_DIR/fips" ]; then
     echo "Error: FIPS binary not found at $DOCKER_DIR/fips" >&2
     echo "Run testing/scripts/build.sh first" >&2
diff --git a/testing/ci-cleanup.sh b/testing/ci-cleanup.sh
index fbcb5ec..0f25243 100755
--- a/testing/ci-cleanup.sh
+++ b/testing/ci-cleanup.sh
@@ -247,6 +247,27 @@ reap_images() {
     timeout "$TMO" docker rmi -f "${imgs[@]}" >/dev/null 2>&1 || true
 }
 
+# Per-run build contexts left in the working tree. ci-local.sh removes its own
+# from the EXIT trap, but the CI worker sends SIGKILL after SIGTERM and a
+# SIGKILL runs no trap, so a preempted run can leave an 18 MB directory behind
+# with nothing else that would ever notice it.
+#
+# Scoped mode takes only the named run's. Broad mode cannot tell a live run's
+# context from an abandoned one by name, so it goes by age instead: a run lasts
+# well under an hour, and a day is far outside that.
+reap_build_contexts() {
+    local dir
+    if [[ -n "$RUN_ID" ]]; then
+        dir="$SCRIPT_DIR/docker-$RUN_ID"
+        [[ -d "$dir" ]] && rm -rf "$dir"
+        return 0
+    fi
+    while IFS= read -r dir; do
+        [[ -n "$dir" ]] && rm -rf "$dir"
+    done < <(find "$SCRIPT_DIR" -maxdepth 1 -type d -name 'docker-*' -mtime +0 2>/dev/null)
+    return 0
+}
+
 # Order matters: containers reference networks/volumes, so drop them first, and
 # the veth sweep needs an image to run ip(8) in, so it precedes the image reap.
 reap_containers
@@ -254,5 +275,6 @@ reap_networks
 reap_volumes
 reap_veths
 reap_images
+reap_build_contexts
 
 exit 0
diff --git a/testing/ci-local.sh b/testing/ci-local.sh
index 3fe7796..670795e 100755
--- a/testing/ci-local.sh
+++ b/testing/ci-local.sh
@@ -318,6 +318,14 @@ CI_LABEL_RUN="com.corganlabs.fips-ci.run=${CI_RUN_ID}"
 export FIPS_CI_RUN_ID="$CI_RUN_ID"
 export FIPS_TEST_IMAGE="$CI_IMAGE_TEST"
 export FIPS_TEST_APP_IMAGE="$CI_IMAGE_APP"
+# The build context is this run's too. testing/docker/ is a single directory in
+# the working tree, so two runs racing on its CONTENTS produce a per-run-tagged
+# image built from the other run's binaries — scoping the tag alone does not
+# close that. Absolute, and it has to be: compose interpolates this into
+# build.context but resolves a relative result against the compose FILE's
+# directory, not the working directory.
+CI_BUILD_CONTEXT="$SCRIPT_DIR/docker-${CI_RUN_ID}"
+export FIPS_BUILD_CONTEXT="$CI_BUILD_CONTEXT"
 # Docker container names are GLOBAL — a compose project name does not scope
 # them — so the suite compose files append this suffix to every explicit
 # container_name, and the suite scripts append it wherever they address a
@@ -405,6 +413,15 @@ ci_teardown() {
         rm -rf "$SCRIPT_DIR/static/generated-configs${CI_RUN_NAME_SUFFIX}"
         rm -rf "$SCRIPT_DIR/firewall/generated-configs${CI_RUN_NAME_SUFFIX}"
     fi
+
+    # 4. This run's build context. Unlike the generated configs it is removed
+    #    on a red run too: it holds the binaries and the Dockerfiles, both
+    #    reproducible from the commit, so it is never the evidence of a
+    #    failure. Guarded on the path having been derived at all, so an early
+    #    exit cannot turn this into `rm -rf $SCRIPT_DIR/docker-`.
+    if [[ -n "${CI_BUILD_CONTEXT:-}" && "$CI_BUILD_CONTEXT" != "$SCRIPT_DIR/docker-" ]]; then
+        rm -rf "$CI_BUILD_CONTEXT"
+    fi
 }
 
 on_signal() {
@@ -871,22 +888,39 @@ run_tor_directory() {
 run_integration() {
     stage "Stage 3: Integration Tests"
 
-    # Install binaries to shared docker context
+    # Populate THIS run's build context, then install the binaries into it.
+    # Everything but the binaries is copied from the tracked context directory;
+    # the binaries are installed fresh, and a previous run's are deliberately
+    # not carried over, since inheriting them is the failure this scoping
+    # exists to prevent.
+    info "Preparing build context $CI_BUILD_CONTEXT"
+    rm -rf "$CI_BUILD_CONTEXT"
+    mkdir -p "$CI_BUILD_CONTEXT" || { record "docker-build" 1; return; }
+    local _f
+    for _f in "$SCRIPT_DIR"/docker/*; do
+        case "$(basename "$_f")" in
+            fips|fipsctl|fipstop|fips-gateway) continue ;;
+        esac
+        cp -a "$_f" "$CI_BUILD_CONTEXT/" || { record "docker-build" 1; return; }
+    done
+
     info "Installing release binaries"
-    install_binaries testing/docker
+    install_binaries "$CI_BUILD_CONTEXT"
 
     # Build unified test image once (used by all harnesses). Tag per-run
     # (fips-test:${run}) so a build killed mid-flight never wedges the next
     # run's rebuild, and concurrent runs never clobber each other's image.
-    # Then retag :latest for the compose files / harness scripts that still
-    # reference fips-test:latest directly; the retag happens only after BOTH
-    # builds succeed, so :latest never points at a half-built image.
     info "Building $CI_IMAGE_TEST Docker image"
-    docker build -t "$CI_IMAGE_TEST" --label "$CI_LABEL" --label "$CI_LABEL_RUN" testing/docker --quiet \
+    docker build -t "$CI_IMAGE_TEST" --label "$CI_LABEL" --label "$CI_LABEL_RUN" "$CI_BUILD_CONTEXT" --quiet \
         || { record "docker-build" 1; return; }
     docker build -t "$CI_IMAGE_APP" --label "$CI_LABEL" --label "$CI_LABEL_RUN" \
-        -f testing/docker/Dockerfile.app testing/docker --quiet \
+        -f "$CI_BUILD_CONTEXT/Dockerfile.app" "$CI_BUILD_CONTEXT" --quiet \
         || { record "docker-build-app" 1; return; }
+    # The remaining bridge back to the shared mutable tag, for any consumer not
+    # yet reading FIPS_TEST_IMAGE. Removed in the following commit, which is the
+    # step that makes a missed consumer fail loudly instead of silently
+    # resolving another run's binaries. Both builds have succeeded by here, so
+    # :latest never points at a half-built image.
     docker tag "$CI_IMAGE_TEST" fips-test:latest
     docker tag "$CI_IMAGE_APP" fips-test-app:latest
 
diff --git a/testing/firewall/docker-compose.yml b/testing/firewall/docker-compose.yml
index 9cedf41..fe65773 100644
--- a/testing/firewall/docker-compose.yml
+++ b/testing/firewall/docker-compose.yml
@@ -19,7 +19,11 @@ networks:
 
 x-fips-common: &fips-common
   build:
-    context: ../docker
+    # The harness scopes its build context per run and passes it here; the
+    # shared directory is the hand-run default. Compose resolves a relative
+    # value against THIS file's directory, so the harness must export an
+    # absolute path.
+    context: ${FIPS_BUILD_CONTEXT:-../docker}
   image: ${FIPS_TEST_IMAGE:-fips-test:latest}
   entrypoint: ["/usr/local/bin/entrypoint.sh"]
   cap_add:
diff --git a/testing/sidecar/scripts/test-sidecar.sh b/testing/sidecar/scripts/test-sidecar.sh
index 2cbd114..975973f 100755
--- a/testing/sidecar/scripts/test-sidecar.sh
+++ b/testing/sidecar/scripts/test-sidecar.sh
@@ -115,7 +115,7 @@ trap cleanup EXIT
 
 if [[ "${1:-}" != "--skip-build" ]]; then
     log "Building test images..."
-    DOCKER_DIR="$(cd "$SIDECAR_DIR/../docker" && pwd)"
+    DOCKER_DIR="${FIPS_BUILD_CONTEXT:-$(cd "$SIDECAR_DIR/../docker" && pwd)}"
     docker build -t fips-test:latest "$DOCKER_DIR"
     docker build -t fips-test-app:latest -f "$DOCKER_DIR/Dockerfile.app" "$DOCKER_DIR"
 fi