Support configuring `.dockerignore` path + support with remote contexts

Description

Better support a git repo as remote context would be ideal with the ability to:

• Adjust the implicit Dockerfile default to another filename (presently only supported via different sub-directories or slightly different --file input?)
• Use the .dockerignore (which would be applied prior to using a shallow git clone as build context?)

Current situation:

# Git URL context => Clones repo into a build context (local or repo `.dockerignore` is not applied)
# `--file` option provides HTTPS URL to select different Dockerfile variant
docker build \
  --tag localhost/project-name:branch-or-tag \
  --file https://raw.githubusercontent.com/user/repo/branch-or-tag/Dockerfile.alpine \
  https://github.com/user/repo.sh.git#branch-or-tag

Proposed:

# --ignore should technically not be required to use `.dockerignore`
# from the remote context unless choosing to opt-out via `/dev/null`?
docker build \
  --tag localhost/project-name:branch-or-tag \
  --file context://Dockerfile.alpine \
  --ignore context://.dockerignore \
  https://github.com/user/repo.sh.git#branch-or-tag

EDIT(Dec 2025): Apparently --file Dockerfile.alpine works now relative to the remote context. I'm lacking information regarding my prior notes above for what happened without that. I know that with a local context that --file is not relative to the build context, so local vs remote contexts do differ there at least for --file.

Additional reference

Remote build contexts and Dockerfile (either implicit via remote context, or explicit via separate --file arg) are supported but not remote .dockerignore files (or even custom path locally).

• When looking into using remote contexts and a non-default Dockerfile file name, I was curious if I could perform a build with a single docker build command and references to a repo on Github, but noticed the larger image from lack of .dockerignore being applied to the build context (which I later noticed is also the case if this file exists locally).
• The official docs still imply .dockerignore would be supported when it exists in the build context root, but is incorrect and needs to update the docs.
• Additional build-context args are supported via CLI, but not one for overriding the default via path value instead of hard-coded filename patterns?

Related:

• https://github.com/docker/buildx/issues/2353#issuecomment-2018378945 (.dockerignore doesn't work for remote build contexts, even when it exists locally)
• https://github.com/docker/buildx/issues/3121#issuecomment-2853151812 (original comment of mine as motivation for feature request)

Start with the docker build/buildx handling for remote Git contexts and the --file, --ignore, and --build-context entry points; the payload names no source files or tests. Compare the behavior described in related issues and the Docker documentation. Done means a remote context can select a custom Dockerfile and apply a chosen .dockerignore path, with documentation matching the implemented behavior.