chore(build-system): use system architecture instead of hardcoded amd64 value

gruntfile.js

@@ -1,6 +1,9 @@
 var autoprefixer = require('autoprefixer');
 var cssnano = require('cssnano');
 var loadGruntTasks = require('load-grunt-tasks');
+var os = require('os');
+var arch = os.arch();
+if ( arch === 'x64' ) arch = 'amd64';
 
 module.exports = function (grunt) {
 
@@ -34,8 +37,8 @@ module.exports = function (grunt) {
   grunt.registerTask('build', [
     'config:dev',
     'clean:app',
-    'shell:buildBinary:linux:amd64',
-    'shell:downloadDockerBinary:linux:amd64',
+    'shell:buildBinary:linux:' + arch,
+    'shell:downloadDockerBinary:linux:' + arch,
     'vendor:regular',
     'html2js',
     'useminPrepare:dev',
@@ -184,7 +187,7 @@ module.exports = function (grunt) {
       run: {
         command: [
           'docker rm -f portainer',
-          'docker run -d -p 9000:9000 -v $(pwd)/dist:/app -v /tmp/portainer:/data -v /var/run/docker.sock:/var/run/docker.sock:z --name portainer portainer/base /app/portainer-linux-amd64 --no-analytics -a /app'
+          'docker run -d -p 9000:9000 -v $(pwd)/dist:/app -v /tmp/portainer:/data -v /var/run/docker.sock:/var/run/docker.sock:z --name portainer portainer/base /app/portainer-linux-' + arch + ' --no-analytics -a /app'
         ].join(';')
       },
       downloadDockerBinary: {