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docs/builder: add note about handling of leading whitespace 

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
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Sebastiaan van Stijn 2020-07-01 15:26:06 +02:00
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docs/reference/builder.md
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@ -173,7 +173,7 @@ be UPPERCASE to distinguish them from arguments more easily.
Docker runs instructions in a `Dockerfile` in order. A `Dockerfile` **must Docker runs instructions in a `Dockerfile` in order. A `Dockerfile` **must
begin with a \`FROM\` instruction**. This may be after [parser begin with a `FROM` instruction**. This may be after [parser
directives](#parser-directives), [comments](#format), and globally scoped directives](#parser-directives), [comments](#format), and globally scoped
[ARGs](#arg). The `FROM` instruction specifies the [*Parent [ARGs](#arg). The `FROM` instruction specifies the [*Parent
Image*](https://docs.docker.com/glossary/#parent_image) from which you are Image*](https://docs.docker.com/glossary/#parent_image) from which you are
@ -189,8 +189,52 @@ else in a line is treated as an argument. This allows statements like:
RUN echo 'we are running some # of cool things' RUN echo 'we are running some # of cool things'
``` ```
Comment lines are removed before the Dockerfile instructions are executed, which
means that the comment in the following example is not handled by the shell
executing the `echo` command, and both examples below are equivalent:
```dockerfile
RUN echo hello \
# comment
world
```
```dockerfile
RUN echo hello \
world
```
Line continuation characters are not supported in comments. Line continuation characters are not supported in comments.
> **Note on whitespace**
>
> For backward compatibility, leading whitespace before comments (`#`) and
> instructions (such as `RUN`) are ignored, but discouraged. Leading whitespace
> is not preserved in these cases, and the following examples are therefore
> equivalent:
>
> ```dockerfile
> # this is a comment-line
> RUN echo hello
> RUN echo world
> ```
>
> ```dockerfile
> # this is a comment-line
> RUN echo hello
> RUN echo world
> ```
>
> Note however, that whitespace in instruction _arguments_, such as the commands
> following `RUN`, are preserved, so the following example prints ` hello world`
> with leading whitespace as specified:
>
> ```dockerfile
> RUN echo "\
> hello\
> world"
> ```
## Parser directives ## Parser directives
Parser directives are optional, and affect the way in which subsequent lines Parser directives are optional, and affect the way in which subsequent lines