Make fig run COMMAND parameter optional.

This behaves more like the native docker client, where the absence of a
command means docker runs the CMD in the Dockerfile. If a command is
defined in fig.yml this is used instead.

Signed-off-by: Chris Corbyn <chris@w3style.co.uk>

tests/fixtures/commands-figfile/fig.yml

@@ -0,0 +1,5 @@
+implicit:
+  image: figtest_test
+explicit:
+  image: figtest_test
+  command: [ "/bin/true" ]