The .dockerignore file has been around for a while, but there's still room for improvement. Opening this as a ticket, based on some notes I had, and some existing issues. I can break down this issue further (and open separate tickets where needed)

Some related issues:

• Verify parity between .dockerignore and .gitignore syntax  #40318 Verify parity between .dockerignore and .gitignore syntax (duplicated by .dockerignore does not recursively include with negated exclusion docker/cli#2847 .dockerignore does not recursively include with negated exclusion)
• Exception patterns in .dockerignore do not support wildcard directories #30018 Exception patterns in .dockerignore do not support wildcard directories
• wildcard directories not implemented correctly in dockerignore #41433 wildcard directories not implemented correctly in dockerignore
• Global .dockerignore #12843 Global .dockerignore
• (re)consider nested .dockerignore files Proposal: recursive .dockerignore files #20944, and Added recursive .dockerignore capabilities #21020 (comment)
• Output the reason while cache need to be invalidated on docker build #9294 "Output the reason while cache need to be invalidated on docker build"
• COPY with excluded files is not possible #15771 COPY with excluded files is not possible
• .dockerignore with '*' wildcard doesn't work as documented docker/for-linux#585 .dockerignore with '*' wildcard doesn't work as documented

Assorted issues / suggestions / topics

• Improve performance where possible. Ignoring files should make builds smaller and faster, not cause builds to be slow.
  • Would be great if it's possible to profile builds, and see how much time was taken by excluding files. Having this would allow users to make better informed decisions wether or not .dockerignore is the right solution.
  • The dockerignore file is often used to make the build-context smaller in order to speed up builds, but with the introduction of BuildKit, some of those may no longer be relevant (and .dockerignore could actually have a negative impact in some situations)
• Make .dockerignore follow the exact same patterns as .gitignore (which it was originally modeled after)
  • .gitignore pattern reference: https://git-scm.com/docs/gitignore#_pattern_format
  • if possible, run the .git test-suite / tests to verify this
• Add a global .dockerignore to allow excluding common files (.idea, Thumbs.db files and such). See [Global .dockerignore #12843](Global .dockerignore #12843 Global .dockerignore)
  • allow easy discovery why a file or directory was ignored (which becomes more important if a global ignore file is active). Also see next bullet
• Introduce a command similar to git check-ignore for debugging
  • related: Output the reason while cache need to be invalidated on docker build #9294 "Output the reason while cache need to be invalidated on docker build"
• Note: recursive .dockerignore files were proposed at some point, but were considered a risky, breaking change (see Proposal: recursive .dockerignore files #20944, and Added recursive .dockerignore capabilities #21020 (comment))
  • should this be reconsidered?
• Improve and validate the .dockerignore documentation
  • document the per-Dockerfile dockerignore (when using BuildKit and Docker 19.03 or up: Add support for specifying .dockerignore file with -i/--ignore #12886 (comment), Add support for specifying .dockerignore file with -i/--ignore #12886 (comment))
  • improve docs by referring to / borrowing from .gitignore (where applicable) (see Verify parity between .dockerignore and .gitignore syntax  #40318 "BUG: .dockerignore sometimes ignores **")
  • could be moved to a separate page if needed
  • provide better examples (also for Windows platforms)
  • best practices (what files to consider for excluding to optimize caching; when to use .dockerignore, when not to use and (e.g.) use BuildKit, optimize your Dockerfile, use multi-stage builds etc.)
  • common pitfalls
• (related) consider implementing COPY --exclude to allow specifying ignore-rules in the Dockerfile itself; this can help with situations where (e.g.) frequently changing files are mixed with "cacheable" files (proposal in COPY with excluded files is not possible #15771). Allowing such files to be COPY'd separately would give more control over layers/caching.
• (Slightly related) improve errors produced due to files that were excluded. The current error for missing files is confusing (as it mentions the file path on the daemon side), therefore making it hard to trace the error back to .dockerignore (see the discussion on /var/lib/docker/tmp/docker-builderXXXXXXX/... no such file or directory docker/for-mac#1922, and need more relevant directory in failure message when Dockerfile is missing buildkit#4248)
• Feature: improve debugging of COPY failures docker/cli#2690
• Improve error messages for non-existing or excluded files buildkit#1647
• Unable to unignore using wildcards with .dockerignore #23693