Give each local CI run its own docker build context

Scoping the image tag was never sufficient on its own. Every build read one
unscoped directory in the working tree, into which each run copies the
binaries it just built, so two concurrent runs raced on the contents of the
context as well as on the name of the result — and a run could produce a
correctly-per-run-tagged image built from the other run's binaries.

The run now copies the context's tracked files into its own directory,
installs its binaries there, and builds from it, exporting the path so every
other consumer follows. Deliberately not carried over: a previous run's
binaries, since inheriting them is the failure this prevents. The path is
absolute because compose resolves a relative build context against the
compose file's own directory rather than the working directory, which was
measured rather than assumed.

The chaos entry script gated the whole simulation on a binary in the shared
directory by literal path, so it moves in this same commit: left behind, it
would have failed on a clean checkout and, worse on a host with leftovers,
passed while reading a binary that was not the one under test.

Teardown removes the directory on red runs as well as green, since it holds
only reproducible content and is never the evidence of a failure. The
worker's SIGKILL runs no trap, so the cleanup script also sweeps contexts
left by a preempted run, and the ignore rule keeps a concurrent run from
showing up as untracked working-tree noise.

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