Clarify .dockerignore usage in README

[npwolf]

The previous instructions made me think mutation of the .dockerignore file was not supported with the git context, but the the basic .dockerignore was supported (not the case). Just trying to improve the doc so it is more clear.

[crazy-max]

but the the basic .dockerignore was supported (not the case)

Not sure I understand this statement. Do you mean .dockerignore file within a Git context is supported?

[crazy-max]

I think this is already self-explanatory that .dockerignore is ignored with Git context here. We also say that Path context should be used in next sentence if you don't want this behavior.

[npwolf]

@crazy-max I personally thought the text was saying file mutations in general and file mutations of the .dockerignore file specifically weren't supported with git context (not that .dockerignore wasn't supported at all in git context). I've updated this with another revision to try to improve the clarity for others. Let me know if I can improve it further. If you don't feel it warrants an update, feel free to close the PR. Thanks!

[mschoettle]

I have a Dockerfile that expects .git to be present. So I ran into this issue with the git vs path context. Once I found the relevant part in the README the explanation there was clear to me.

The problem I had is that I looked in the wrong place of the README (the examples section). So maybe we could add a link to this part of the README in the examples section?

[crazy-max]

I have a Dockerfile that expects .git to be present. So I ran into this issue with the git vs path context. Once I found the relevant part in the README the explanation there was clear to me.

The problem I had is that I looked in the wrong place of the README (the examples section). So maybe we could add a link to this part of the README in the examples section?

You can keep the .git folder with Git context if you set the build-arg BUILDKIT_CONTEXT_KEEP_GIT_DIR=1: https://docs.docker.com/reference/dockerfile/#buildkit-built-in-build-args