Fix panic on cached op with bind mounts

[ktock]

This commit fixes the panic which occurs when execing into the cached op with bind mounts

DEBU[2022-06-28T02:23:02Z] finishing listening debug events             
panic: runtime error: index out of range [1] with length 1

goroutine 43 [running]:
main.(*handler).execContainer(0xc00028bb00, {0x12b1498, 0xc000122000}, {0xc000158f50, {0xc00023b160, 0x1, 0x1}, 0x1, {0x129ec28, 0xc00023b460}, ...})
	/go/src/github.com/ktock/buildg/exec.go:130 +0xd9f
main.execCommand.func1(0xc000c7e420)
	/go/src/github.com/ktock/buildg/exec.go:91 +0x52b
github.com/urfave/cli.HandleAction({0xf5ff00, 0xc0007464c0}, 0x4)
	/go/pkg/mod/github.com/urfave/cli@v1.22.9/app.go:524 +0xa8
github.com/urfave/cli.Command.Run({{0x111d2ab, 0x4}, {0x0, 0x0}, {0xc00023afb0, 0x1, 0x1}, {0x1137617, 0x1b}, {0x116692f, ...}, ...}, ...)
	/go/pkg/mod/github.com/urfave/cli@v1.22.9/command.go:173 +0x652
github.com/urfave/cli.(*App).Run(0xc000bf8540, {0xc000c689f0, 0x3, 0x3})
	/go/pkg/mod/github.com/urfave/cli@v1.22.9/app.go:277 +0x705
main.(*handler).dispatch(0xc00028bb00, {0x12b1498, 0xc000122000}, 0xc000158f50, {0xc000010658, 0x1, 0x1}, {0xc0007464a0, 0x2, 0x2})
	/go/src/github.com/ktock/buildg/handler.go:235 +0x499
main.(*handler).handle(0xc00028bb00, {0x12b1498, 0xc000122000}, 0x12, {0xc000010658, 0x1, 0x1})
	/go/src/github.com/ktock/buildg/handler.go:172 +0x505
main.(*debugController).handle(0xc000650600, {0x12b1498, 0xc000122000}, 0x9e4b00)
	/go/src/github.com/ktock/buildg/debug.go:65 +0x317
main.withDebug.func1.1()
	/go/src/github.com/ktock/buildg/handler.go:87 +0x90
created by main.withDebug.func1
	/go/src/github.com/ktock/buildg/handler.go:84 +0x385