A simple docker build extension that works with .gitignore file directives and you can specify both Dockerfile and multiple .gitignore paths

dockerignore.py

# Version 0.0.1 (works with python2.7 and python3)
# Does not work with non-local context (i.e. git, remote archives)
# You need the pathspec package installed (pip install pathspec)
# Pass any docker build arguments you want to use after -- argument (i.e., dockerignore.py . -- -t blah)
# This works with .gitignore syntax, not .dockerignore ( https://git-scm.com/docs/gitignore#_pattern_format )
import tarfile
import subprocess
import argparse
import os
import sys
try:
    from StringIO import StringIO
except ModuleNotFoundError:
    from io import BytesIO as StringIO

import pathspec


def tar_add_string_as_file(tar, name, s):
    t = tarfile.TarInfo(name=name)
    t.size = len(s)
    tar.addfile(tarinfo=t, fileobj=StringIO(s))


def build_from(dockerfile, context, gitignorefiles, compress, buildparams, verbose):

    def filter(tarinfo):
        if not dockerfile and tarinfo.name == 'Dockerfile':
            return tarinfo
        for ps in pathspecs:
            if ps.match_file(tarinfo.name):
                return
        if verbose:
            print(tarinfo.name)
        return tarinfo

    if not gitignorefiles:
        gitignorefiles = []

    if not os.path.isdir(context):
        raise Exception('context must be a directory: %s' % context)

    pathspecs = []
    for filepath in gitignorefiles:
        with open(filepath, 'rt') as f:
            spec = pathspec.PathSpec.from_lines('gitignore', f)
            pathspecs.append(spec)

    proc = subprocess.Popen(['docker', 'build', '-'] + buildparams, stdin=subprocess.PIPE)
    try:
        with tarfile.open(mode='w|' + compress, fileobj=proc.stdin) as tar:
            tar_add_string_as_file(tar, '.dockerignore', b'Dockerfile\n.dockerignore')
            tar.add(context, arcname='', filter=filter)
            if dockerfile:
                tar.add(dockerfile, arcname='Dockerfile')
        proc.stdin.close()
        return proc.wait()
    except Exception:
        proc.kill()
        raise


def fix_argv():
    try:
        ind = sys.argv.index('--')
        ret = sys.argv[ind+1:]
        sys.argv = sys.argv[:ind]
        return ret
    except ValueError:
        return []


if __name__ == '__main__':
    docker_build_args = fix_argv()
    parser = argparse.ArgumentParser()
    parser.add_argument("context")
    parser.add_argument("-f", "--file", help="Name of the Dockerfile (Default is 'PATH/Dockerfile')")
    parser.add_argument("-i", "--gitignorefiles", action="append", help="a .gitignore file")
    parser.add_argument("--gz", action="store_true", help="Compress the build context using gzip")
    parser.add_argument("--bz2", action="store_true", help="Compress the build context using bzip2")
    parser.add_argument("--verbose", action="store_true", help="Show the files added to the context")
    args = parser.parse_args()
    if args.gz and args.bz2:
        raise Exception('Cannot enable both gzip and bzip2 compression')
    compress = 'gz' if args.gz else 'bz2' if args.bz2 else ''
    sys.exit(build_from(args.file, args.context, args.gitignorefiles, compress, docker_build_args, args.verbose))