Have every suite compose file name the caller's test image

Only the static family read FIPS_TEST_IMAGE; the firewall, ACL, nat,
sidecar and both Tor compose files named the shared mutable tag directly,
which is why the local CI runner has to retag its per-run image to that
name. They now take the same defaulted form the static file already uses,
so a harness run resolves the image it built and a bare hand run still
resolves the shared tag exactly as before.

Checked by rendering each file through `docker compose config` with the
variables set: every service resolves to the supplied name and none is
left naming the shared tag.

testing/nat/docker-compose.yml

@@ -18,7 +18,7 @@ volumes:
   relay-data:
 
 x-fips-common: &fips-common
-  image: fips-test:latest
+  image: ${FIPS_TEST_IMAGE:-fips-test:latest}
   cap_add:
     - NET_ADMIN
   devices:
@@ -325,7 +325,7 @@ services:
         ipv4_address: 172.31.10.51
 
   stun-fault-shim:
-    image: fips-test:latest
+    image: ${FIPS_TEST_IMAGE:-fips-test:latest}
     profiles: ["stun-faults"]
     container_name: fips-nat-stun-fault-shim${FIPS_CI_NAME_SUFFIX:-}
     depends_on: