compose: Use the hostname on the host as default hostname for services with host network mode

Otherwise if the hostname on the supervisor container differs from the hostname on the host, the current and target
services will never match.

Change-Type: patch
Signed-off-by: Pablo Carranza Velez <pablo@resin.io>

src/application-manager.coffee

@@ -799,11 +799,13 @@ module.exports = class ApplicationManager extends EventEmitter
 				firmware: pathExistsOnHost('/lib/firmware')
 				modules: pathExistsOnHost('/lib/modules')
 			})
-			(opts, supervisorApiHost, hostPathExists) =>
+			fs.readFileAsync(path.join(constants.rootMountPoint, '/etc/hostname')).then(String)
+			(opts, supervisorApiHost, hostPathExists, hostnameOnHost) =>
 				configOpts = {
 					appName: app.name
 					supervisorApiHost
 					hostPathExists
+					hostnameOnHost
 				}
 				_.assign(configOpts, opts)
 				volumes = JSON.parse(app.volumes)

src/compose/service.coffee

@@ -257,6 +257,9 @@ module.exports = class Service
 					return "#{@appId}_#{k}"
 				return k
 
+			if @networkMode == 'host' and @hostname == ''
+				@hostname = opts.hostnameOnHost
+
 			@networks[@networkMode] ?= {}
 
 			@restartPolicy = createRestartPolicy(serviceProperties.restart)
@@ -459,9 +462,8 @@ module.exports = class Service
 		hostname = container.Config.Hostname
 		# A hostname equal to the first part of the container ID actually
 		# means no hostname was specified
-		if hostname == container.Id.substr(0, 12) or
-			(networkMode == 'host' and hostname == os.hostname())
-				hostname = ''
+		if hostname.length is 12 and container.Id.startsWith(hostname)
+			hostname = ''
 
 		service = {
 			appId: appId