Shipyard: How do you use '&' in environment variable values?

gistfile1.sh

$ cat add-container.json
{
  "name": "flynn/slugrunner",
  "cpus": 0.1,
  "memory": 256,
  "type": "service",
  "hostname": "",
  "domain": "",
  "labels": ["sample-web"],
  "args": [],
  "environment": {
    "SLUG_URL": "https://example.com/slugs?key1=val1&key2=val2"
  },
  "restart_policy": {
    "name": "always"
  },
  "bind_ports": [
    {
      "host_ip": "10.1.2.3",
      "proto": "tcp",
      "container_port": 8080
    },
    {
      "proto": "tcp",
      "port": 80,
      "container_port": 8080
    }
  ],
  "links": {
  }
}

$ curl -XPOST -H 'Content-Type: application/json' \
     -H 'X-Service-Key: REDACTED' \
     -H 'Accept: application/json' \
     --data @add-container.json \
     http://localhost:8080/api/containers
     
[{"id":"a814121e6426326c065f5ddcdb4e4f78c083de9c596659f8a7e281d497902655","image":{"name":"flynn/slugrunner","cpus":0.1,"memory":256,"environment":{"SLUG_URL":"https://example.com/slugs?key1=val1\u0026key2=val2"},"type":"service","labels":["sample-web"],"bind_ports":[{"proto":"tcp","host_ip":"10.1.2.3","container_port":8080},{"proto":"tcp","port":80,"container_port":8080}],"restart_policy":{"name":"always"}},"engine":{"id":"host1","addr":"http://172.17.42.1:2375","cpus":1,"memory":1024,"labels":["sample-web"]},"ports":[{"proto":"tcp","host_ip":"0.0.0.0","port":80,"container_port":8080}]}]
     

The value for SLUG_URL changes from https://example.com/slugs?key1=val1&key2=val2 to https://example.com/slugs?key1=val1\u0026key2=val2. The version with the \u0026 encoded ampersand is what ends up being passed to docker.