appengine:deploy fails when using alternate app.yaml file name

[mr-smithers-excellent]

I am attempting to change the name of the app.yaml file to create one config for each of our environments. When running mvn appengine:deploy -Dapp.deploy.deployables=src/main/appengine/dev.yaml I get the following error message:

[INFO] Detected App Engine flexible environment application.
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 9.109 s
[INFO] Finished at: 2017-09-28T11:58:40-07:00
[INFO] Final Memory: 28M/331M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal com.google.cloud.tools:appengine-maven-plugin:1.3.1:deploy (default-cli) on project platform-poc: Execution default-cli of goal com.google.cloud.tools:appengine-maven-plugin:1.3.1:deploy failed: app.yaml not found in the App Engine directory. -> [Help 1]
[ERROR] 
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR] 
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/PluginExecutionException

Running with -X, I see can that the plugin has received the param:

[DEBUG] Configuring mojo com.google.cloud.tools:appengine-maven-plugin:1.3.1:deploy from plugin realm ClassRealm[plugin>com.google.cloud.tools:appengine-maven-plugin:1.3.1, parent: sun.misc.Launcher$AppClassLoader@135fbaa4]
[DEBUG] Configuring mojo 'com.google.cloud.tools:appengine-maven-plugin:1.3.1:deploy' with basic configurator -->
[DEBUG]   (f) artifact = /Users/seanpsmith/Projects/aalto-group/platform-poc/target/platform-poc.jar
[DEBUG]   (f) deployables = [/Users/seanpsmith/Projects/aalto-group/platform-poc/src/main/appengine/dev.yaml]
[DEBUG]   (f) dockerDirectory = /Users/seanpsmith/Projects/aalto-group/platform-poc/src/main/docker
[DEBUG]   (f) dockerfilePrimaryDefaultLocation = /Users/seanpsmith/Projects/aalto-group/platform-poc/src/main/docker/Dockerfile
[DEBUG]   (f) dockerfileSecondaryDefaultLocation = /Users/seanpsmith/Projects/aalto-group/platform-poc/src/main/appengine/Dockerfile
[DEBUG]   (f) mavenProject = MavenProject: com.aaltogroup:platform-poc:0.0.1 @ /Users/seanpsmith/Projects/aalto-group/platform-poc/pom.xml
[DEBUG]   (f) pluginDescriptor = Component Descriptor: role: 'org.apache.maven.plugin.Mojo', implementation: 'com.google.cloud.tools.maven.DeployMojo', role hint: 'com.google.cloud.tools:appengine-maven-plugin:1.3.1:deploy'