Support serverless.yml and serverless.yaml

This should fix the issue reported by @mrpgraae in serverless#210.
I've also added support for `SLS_DEBUG` so we can more easily get some information on future errors.

Author: kichik

lib/docker.js

@@ -1,5 +1,7 @@
 const { spawnSync } = require('child_process');
 const isWsl = require('is-wsl');
+const fse = require('fs-extra');
+const path = require('path');
 
 /**
  * Helper function to run a docker command
@@ -32,24 +34,43 @@ function buildImage(dockerFile) {
   return imageName;
 }
 
+/**
+ * Find a file that exists on all projects so we can test if Docker can see it too
+ * @param {string} servicePath
+ * @return {string} file name
+ */
+function findTestFile(servicePath) {
+  if (fse.pathExistsSync(path.join(servicePath, 'serverless.yml'))) {
+    return 'serverless.yml';
+  }
+  if (fse.pathExistsSync(path.join(servicePath, 'serverless.yaml'))) {
+    return 'serverless.yaml';
+  }
+  throw new Error('Unable to find serverless.yml or serverless.yaml for getBindPath()');
+}
+
 /**
  * Test bind path to make sure it's working
  * @param {string} bindPath
  * @return {boolean}
  */
-function tryBindPath(bindPath) {
+function tryBindPath(bindPath, testFile) {
   const options = [
     'run',
     '--rm',
     '-v',
     `${bindPath}:/test`,
     'alpine',
     'ls',
-    '/test/serverless.yml'
+    `/test/${testFile}`
   ];
   try {
     const ps = dockerCommand(options);
-    return ps.stdout.trim() === '/test/serverless.yml';
+    if (process.env.SLS_DEBUG) {
+      console.log(`Trying bindPath ${bindPath} (${options})`)
+      console.log(ps.stdout.trim());
+    }
+    return ps.stdout.trim() === `/test/${testFile}`;
   } catch (err) {
     return false;
   }
@@ -94,15 +115,16 @@ function getBindPath(servicePath) {
 
   bindPaths.push(`/${drive.toLowerCase()}/${path}`); // Docker Toolbox (seems like Docker for Windows can support this too)
   bindPaths.push(`${drive.toLowerCase()}:/${path}`); // Docker for Windows
-  // other options just in case
   bindPaths.push(`/${drive.toUpperCase()}/${path}`);
   bindPaths.push(`/mnt/${drive.toLowerCase()}/${path}`);
   bindPaths.push(`/mnt/${drive.toUpperCase()}/${path}`);
   bindPaths.push(`${drive.toUpperCase()}:/${path}`);
+  
+  const testFile = findTestFile(servicePath);
 
   for (let i = 0; i < bindPaths.length; i++) {
     const bindPath = bindPaths[i];
-    if (tryBindPath(bindPath)) {
+    if (tryBindPath(bindPath, testFile)) {
       return bindPath;
     }
   }