patch: build.go access custom labels directly cause panic

[RiskyFeryansyahP]

This change fixes the panic: assignment to entry in nil map error when want to assigning value to CustomLabel. I think this related to this PR that forgot to change this line code too.

What I did

Related issue
Error that got me here:

panic: assignment to entry in nil map

goroutine 10 [running]:
github.com/docker/compose/v2/pkg/compose.(*composeService).ensureImagesExists(0x1400003d0c0?, {0x1030de670, 0x1400045be90}, 0x1400044e3c0, 0x0)
        /Users/ruangguru/Go/pkg/mod/github.com/docker/compose/[email protected]/pkg/compose/build.go:141 +0x3a4
github.com/docker/compose/v2/pkg/compose.(*composeService).create(0x1400003d0c0?, {0x1030de670, 0x1400045be90}, 0x1400044e3c0, {{0x1400003d100, 0x1, 0x1}, 0x0, 0x0, {0x102c2558c, ...}, ...})
        /Users/ruangguru/Go/pkg/mod/github.com/docker/compose/[email protected]/pkg/compose/create.go:67 +0x114
github.com/docker/compose/v2/pkg/compose.(*composeService).Create.func1({0x1030de670?, 0x1400045be90?})
        /Users/ruangguru/Go/pkg/mod/github.com/docker/compose/[email protected]/pkg/compose/create.go:52 +0x6c
github.com/docker/compose/v2/pkg/progress.Run.func1({0x1030de670?, 0x1400045be90?})
        /Users/ruangguru/Go/pkg/mod/github.com/docker/compose/[email protected]/pkg/progress/writer.go:61 +0x30
github.com/docker/compose/v2/pkg/progress.RunWithStatus.func2()
        /Users/ruangguru/Go/pkg/mod/github.com/docker/compose/[email protected]/pkg/progress/writer.go:82 +0x7c
golang.org/x/sync/errgroup.(*Group).Go.func1()
        /Users/ruangguru/Go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x60
created by golang.org/x/sync/errgroup.(*Group).Go
        /Users/ruangguru/Go/pkg/mod/golang.org/x/[email protected]/errgroup/errgroup.go:72 +0xa8

(not mandatory) A picture of a cute animal, if possible in relation with what you did

[RiskyFeryansyahP]

Hello guys, this is my first PR in here. so, any feedback is welcome 😄

[glours]

Hey 👋
Can you sign-off your work? Just follow the instructions

[RiskyFeryansyahP]

Done, thank you for reminding @glours

[glours]

Thanks for catching this 👍