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_test.go

@@ -45,7 +45,7 @@ func TestShellExecuteCmd(t *testing.T) {
 
 	for _, test := range shellTests {
 		cmd := ShellCommand{
-			&instructions.ShellCommand{
+			cmd: &instructions.ShellCommand{
 				Shell: test.cmdLine,
 			},
 		}