ci: explicitly wire secrets with from_secret in environment

.woodpecker.yml

@@ -38,6 +38,8 @@ steps:
         from_secret: REGISTRY_USERNAME
       REGISTRY_PASSWORD:
         from_secret: REGISTRY_PASSWORD
+      SERVICE_NAME:
+        from_secret: SERVICE_NAME
     commands:
       - echo "${REGISTRY_PASSWORD}" | docker login "${REGISTRY_HOSTPORT}" -u "${REGISTRY_USERNAME}" --password-stdin
       - docker build -t "${REGISTRY_HOSTPORT}/nxtgauge-rust-${SERVICE}" --build-arg SERVICE_NAME=${SERVICE} -f Dockerfile.simple .