Add Dockerfile format

[sbrunner]

Some plugin using this project like:

• https://marketplace.visualstudio.com/items?itemName=foxundermoon.shell-format
• https://github.com/rx-ts/prettier/tree/master/packages/sh

Use this formatter to format Dockerfile and with correct option, the result is not so bad but in prefect...

Isn't possible to support it officially?

The issues I detect:
Removing some ending '; ':

RUN set -eux; \
    false; \
    true

=>

RUN set -eux
false
true

Error with multiline if and for:

RUN if [ "$TEST" == TRUE ]; \
    then \
    echo test; \
    fi

=> error:

"then" can only be used in an if

RUN for item in "${TEST}"; \
    do \
        echo $item; \
    done

=> error:

"do" can only be used in a loop

[mvdan]

Sorry, but this software does not aim to parse or format Dockerfiles. It parses and formats shell. Dockerfiles simply embed bits of shell in them, but the overall syntax is not shell.

A previous occurrence of this issue is #424. cc @JounQin @kaey

My overall thoughts are that someone could easily build a Dockerfile formatter in Go on top of this shell formatter. The closest one that I know of is https://github.com/jessfraz/dockfmt; it does its own simple formatting of shell code, which is why I raised jessfraz/dockfmt#2 a while back. The issue is labeled "help wanted", so I'm sure a PR would be welcome.

[JounQin]

@sbrunner binaryNextLine is auto enabled in prettier-plugin-sh, so actually it's not a problem for prettier-plugin-sh.

[mvdan]

I got no answer to the above, so closing.