lstags: goroutine fatal when pushing big amount of images

in similar context as https://github.com/ivanilves/lstags/issues/153, but different scenario, got goroutine fatal errors when pushing ~400 images to registry.

the registry:5000 is insecure registry confured via env: INSECURE_REGISTRY_EX=registry:5000

worked fine with 1 image with same configuration.

$ lstags '--docker-json=/root/.docker/config.json' '--yaml-config=lstags.yml' --push '--push-prefix=/' '--push-registry=registry:5000'
INFO[0000] BATCH 1 of 13
fatal error: concurrent map writes

goroutine 1346 [running]:
runtime.throw(0x7f16c1, 0x15)
    /home/travis/.gimme/versions/go1.10.linux.amd64/src/runtime/panic.go:619 +0x81 fp=0xc4203d5d60 sp=0xc4203d5d40 pc=0x429951
runtime.mapassign_faststr(0x77a200, 0xc42008b650, 0xc42030a1b0, 0x2f, 0xc420f91d40)
    /home/travis/.gimme/versions/go1.10.linux.amd64/src/runtime/hashmap_fast.go:703 +0x3e9 fp=0xc4203d5dd0 sp=0xc4203d5d60 pc=0x40bb39
github.com/ivanilves/lstags/api/v1.(*API).CollectTags.func1(0xc4202c6320, 0xc42008b650, 0xc420d9a9c0, 0xc420e339e0)
    /home/travis/gopath/src/github.com/ivanilves/lstags/api/v1/v1.go:143 +0x609 fp=0xc4203d5fc0 sp=0xc4203d5dd0 pc=0x6f0709
runtime.goexit()
    /home/travis/.gimme/versions/go1.10.linux.amd64/src/runtime/asm_amd64.s:2361 +0x1 fp=0xc4203d5fc8 sp=0xc4203d5fc0 pc=0x455791
created by github.com/ivanilves/lstags/api/v1.(*API).CollectTags
    /home/travis/gopath/src/github.com/ivanilves/lstags/api/v1/v1.go:120 +0x24f

More complete output: push_b_1a.log