File permissions and long startup time

Based on !5 (closed). This MR has been opened to fix review feedback while maintaining the commit authorship to the original contributer.

There are two fixes here. The first sets USER for herokuish build so that the files it generates are under the correct user. Otherwise the files are owned by a randomly generated unprivileged user:

Step 5/11 : RUN ls -la /app
 ---> Running in 8e53243e29bd
total 84
drwxr-xr-x 10 u16271 u16271  4096 May  1 00:46 .
drwxr-xr-x  1 root   root    4096 May  1 00:46 ..
-rw-r--r--  1 u16271 u16271   315 May  1 00:45 Dockerfile
-rw-r--r--  1 u16271 u16271    52 May  1 00:45 Gemfile
-rw-r--r--  1 u16271 u16271   150 May  1 00:45 Gemfile.lock
-rw-r--r--  1 root   root      91 May  1 00:46 Procfile
-rw-r--r--  1 u16271 u16271    79 May  1 00:45 Rakefile
drwxr-xr-x  2 u16271 u16271  4096 May  1 00:46 bin
-rwxr-xr-x  1 u16271 u16271  1085 May  1 00:45 build.sh
drwxr-xr-x  3 u16271 u16271  4096 May  1 00:45 chart
-rw-r--r--  1 u16271 u16271   121 May  1 00:45 config.ru
drwxr-xr-x  2 u16271 u16271  4096 May  1 00:46 tmp
drwxr-xr-x  5 u16271 u16271  4096 May  1 00:46 vendor

This is probably enough to fix startup times on its own, but it does leave a few files owned by root, Procfile for example. So for belt and braces when the built files are copied to the final image they are also chown'd.

Fixes #4 (closed)