// 栈不为空时才判断（避免访问stack.at(-1)时报错）
When you write a Dockerfile, the Dockerfile frontend parses it and emits LLB. But nothing in BuildKit requires that the input be a Dockerfile. Any program that can produce valid LLB can drive BuildKit.
，详情可参考谷歌浏览器【最新下载地址】
Disproportionate impact on marginalized communities and controversial but legal applications
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