title: Drone replacing environment variables with empty strings
body: <p>Something happened with my Drone configuration. It's not finding the environment variables since today. Until few days ago, I could run a pipeline, but today I can't.</p>
  
  <p>This is the step into the pipeline:</p>
  
  <pre><code>pipeline: 
      [...]
      sdk:
          image: mycompany/swagger-codegen:latest
          environment:
              - API_SWAGGER_JSON_URL=http://api.mycompany.biz:9000/v1/swagger.json
              - API_PACKAGE=com.mycompany.api
              - API_GROUP_ID=com.mycompany.api
              - API_ARTIFACT_ID=sdk
              - API_VERSION=0.1-SNAPSHOT
          when:
              branch: master
          commands:
              - java -jar /usr/lib/swagger/swagger-codegen-cli.jar generate
                  -i ${API_SWAGGER_JSON_URL}
                  --api-package ${API_PACKAGE}
                  --invoker-package ${API_PACKAGE}.client
                  --model-package ${API_PACKAGE}.client.model
                  --group-id ${API_GROUP_ID}
                  --artifact-id ${API_ARTIFACT_ID}
                  --artifact-version ${API_VERSION}
                  -l java
                  -o ./swagger-codegen-source
              - etc.
  </code></pre>
  
  <p>And this is what I get </p>
  
  <pre><code>+ java -jar /usr/lib/swagger/swagger-codegen-cli.jar generate -i --api-package --invoker-package .client --model-package .client.model --group-id --artifact-id --artifact-version -l java -o ./swagger-codegen-source
  Exception in thread "main" io.airlift.airline.ParseArgumentsUnexpectedException: Found unexpected parameters: [java]
      at io.airlift.airline.Cli.validate(Cli.java:148)
      at io.airlift.airline.Cli.parse(Cli.java:116)
      at io.airlift.airline.Cli.parse(Cli.java:97)
      at io.swagger.codegen.SwaggerCodegen.main(SwaggerCodegen.java:36)
  </code></pre>
  
  <p>Look at the command. <strong>Every environment variable was substituted by an empty string</strong>. Am I doing something wrong?</p>