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# tools/docker/scala.bzl
load(
    "@io_bazel_rules_docker//lang:image.bzl",
    "app_layer",
)


def scala_image(
    base = "@vertis_openjdk17//image"
...
):

 for index, dep in enumerate(jar_layers):
    base = app_layer(name = "%s.%d" % (name, index), base = base, dep = dep, tags = tags)

 # и дальше в сам рул (java_binary_image) прокидывается этот base слой