fix the compose call: it never worked outside --dry-run

Setting IFS to a newline to pass the environment also stopped the command
line from splitting, so '-f a -f b' went out as a single argument and the two
words 'docker compose' were looked up as one program name. Every real command
was broken; only --dry-run, which prints a string, looked right.

The variables are now exported in a subshell instead of going through env(1),
which keeps values with spaces intact and leaves the command line to normal
word splitting.

The tests only checked what --dry-run printed, which is why they missed it.
spec/unit/compose_spec.sh now asserts the argv that is actually executed, for
both the plugin and the standalone binary.

spec/support/bin/docker

@@ -1,6 +1,10 @@
 #!/bin/sh
 # Mock docker for myos tests: logs every call, answers a few read-only queries deterministically.
-[ -n "${MYOS_DOCKER_LOG:-}" ] && printf 'docker %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+if [ -n "${MYOS_DOCKER_LOG:-}" ]; then
+  printf 'docker %s\n' "$*" >> "$MYOS_DOCKER_LOG"
+  # the environment is recorded too, so a test can assert what was exported
+  env > "$(dirname "$MYOS_DOCKER_LOG")/env.log"
+fi
 case "$1 $2" in
   "compose version") echo "${MOCK_COMPOSE_VERSION:-2.29.0}" ;;
   "compose config")  echo "# mock compose config" ;;