Add .dockerignore to slim down builds a bit

.dockerignore

@@ -0,0 +1,18 @@
+# Object files
+*.o
+# Editor backups and recovery files
+*~
+\#*#
+# build directories
+*.386-x/**
+*.386/**
+*.sparc-x/**
+*.sparc/**
+*.x86_64-x/**
+*.x86_64/**
+*.armv7l-x/**
+*.armv7l/**
+init.386/**
+# core files
+core
+*.core