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

[polarathene]

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

---

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:

• Buildkit ignores .dockerignore in Git repository #2353 (comment) (.dockerignore doesn't work for remote build contexts, even when it exists locally)
• Remote Dockerfiles #3121 (comment) (original comment of mine as motivation for feature request)