command

$ docker pull datadog/agent:latest
latest: Pulling from datadog/agent
Digest: sha256:c47fd9b4eda5fab58835c2dcf6dd6590b69972c9398a6eab84fef0858a6e2e2c
Status: Image is up to date for datadog/agent:latest
$ docker build -t dd . && docker run --rm --name tomtest -h tomtest -e DD_API_KEY=<key> dd | grep hello
Sending build context to Docker daemon  4.096kB
Step 1/3 : FROM datadog/agent:latest
 ---> aa7ad467af17
Step 2/3 : ADD hello.yaml /etc/datadog-agent/conf.d/hello.yaml
 ---> Using cache
 ---> 04e482b3c333
Step 3/3 : ADD hello.py /etc/datadog-agent/checks.d/hello.py
 ---> Using cache
 ---> 2e98ff9f87e5
Successfully built 2e98ff9f87e5
Successfully tagged dd:latest
[ AGENT ] 2018-11-08 15:22:12 UTC | WARN | (file.go:180 in collectEntry) | /etc/datadog-agent/conf.d/hello.yaml is not a valid config file: Configuration file contains no valid instances

Dockerfile

FROM datadog/agent:latest
ADD hello.yaml /etc/datadog-agent/conf.d/hello.yaml
ADD hello.py /etc/datadog-agent/checks.d/hello.py

hello.py

__version__ = "1.0.0"
from checks import AgentCheck
class HelloCheck(AgentCheck):
    def check(self, instance):
        self.gauge('hello.world', 1)

hello.yaml

instances: [{}]
init_config:
  instances:
    - min_collection_interval: 30