Rework cache key generation a bit. (#375)

* Rework cache key generation a bit.

Cache keys are now based on the previous commands, rather than the previous state
of the filesystem.

* Refactor command interface a bit, only cache the context for commands that use it.

pkg/commands/shell.go

@@ -23,6 +23,7 @@ import (
 )
 
 type ShellCommand struct {
+	BaseCommand
 	cmd *instructions.ShellCommand
 }
 
@@ -32,17 +33,7 @@ func (s *ShellCommand) ExecuteCommand(config *v1.Config, buildArgs *dockerfile.B
 	return nil
 }
 
-// FilesToSnapshot returns an empty array since this is a metadata command
-func (s *ShellCommand) FilesToSnapshot() []string {
-	return []string{}
-}
-
 // String returns some information about the command for the image config history
 func (s *ShellCommand) String() string {
 	return s.cmd.String()
 }
-
-// CacheCommand returns false since this command shouldn't be cached
-func (s *ShellCommand) CacheCommand() bool {
-	return false
-}