Fix libffi build failure for the main Dockerfile

This PR changes the main Dockerfile so that it's using libffi-dev to avoid having to build libffi from sources.

This fixes the following error I've run into:
```
ERROR:  Error installing fpm:
        ERROR: Failed to build gem native extension.
 
        /usr/bin/ruby1.8 extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include,/usr/include/ffi... no
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_with_gvl()... no
checking for rb_thread_call_without_gvl()... no
checking for ffi_prep_cif_var()... no
creating extconf.h
creating Makefile
 
make
Configuring libffi
pwd: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory
configure: error: working directory cannot be determined
make: *** ["/var/lib/gems/1.8/gems/ffi-1.9.0/ext/ffi_c/libffi-x86_64-linux"/.libs/libffi_convenience.a] Error 1
 
 
Gem files will remain installed in /var/lib/gems/1.8/gems/ffi-1.9.0 for inspection.
Results logged to /var/lib/gems/1.8/gems/ffi-1.9.0/ext/ffi_c/gem_make.out
Building native extensions.  This could take a while...
Building native extensions.  This could take a while...
Building native extensions.  This could take a while...
Error build: The command [/bin/sh -c gem install fpm] returned a non-zero code: 1
The command [/bin/sh -c gem install fpm] returned a non-zero code: 1
```