参考教程：https://docs.docker.com/engine/reference/builder/
环境
- virtual box 6.1
- centos 7.8
- docker 19.03
解析器指令
Parser directives are optional, and affect the way in which subsequent lines in a Dockerfile are handled. Parser directives do not add layers to the build, and will not be shown as a build step. Parser directives are written as a special type of comment in the form # directive=value. A single directive may only be used once.
解析器指令是可选的，并且会影响 Dockerfile 中后续行的处理方式。解析器指令不会在构建中添加镜像层，也不会显示为构建步骤。解析器指令以 # directive=value 的形式写为特殊的注释类型。单个指令只能使用一次。
Once a comment, empty line or builder instruction has been processed, Docker no longer looks for parser directives. Instead it treats anything formatted as a parser directive as a comment and does not attempt to validate if it might be a parser directive. Therefore, all parser directives must be at the very top of a Dockerfile.
处理完注释，空行或生成器指令后，Docker 不再寻找解析器指令。而是将格式化为解析器指令的任何内容都视为注释，并且不会尝试验证它是否可能是解析器指令。因此，所有解析器指令必须位于 Dockerfile 的最顶部。
Parser directives are not case-sensitive. However, convention is for them to be lowercase. Convention is also to include a blank line following any parser directives. Line continuation characters are not supported in parser directives.
解析器指令不区分大小写。但是，约定是小写的。约定还应在任何解析器指令之后包含一个空白行。解析器伪指令不支持行继续符。
Due to these rules, the following examples are all invalid:
由于这些规则，以下示例均无效：
Invalid due to line continuation:
由于行继续而无效：
# direc
tive=value
Invalid due to appearing twice:
由于出现两次而无效：
# directive=value1
# directive=value2
FROM ImageName
Treated as a comment due to appearing after a builder instruction:
由于在构建指令之后出现，因此被视为注释：
FROM ImageName
# directive=value
Treated as a comment due to appearing after a comment which is not a parser directive:
由于出现在不是解析器指令的注释之后，因此被视为注释：
# About my dockerfile
# directive=value
FROM ImageName
The unknown directive is treated as a comment due to not being recognized. In addition, the known directive is treated as a comment due to appearing after a comment which is not a parser directive.
由于未被识别，未知指令被视为注释。另外，由于在非解析程序指令的注释之后出现，因此已知指令被视为注释。
# unknowndirective=value
# knowndirective=value
Non line-breaking whitespace is permitted in a parser directive. Hence, the following lines are all treated identically:
解析器指令中允许非换行空格。因此，以下各行都被相同地对待：
#directive=value
# directive =value
# directive= value
# directive = value
# dIrEcTiVe=value
The following parser directives are supported:
支持以下解析器指令：
syntaxescape
总结
介绍了 Dockerfile 的解析器指令格式。