.dockerignore can't deal with filenames with "+" in them

Description

Neither ignoring nor unignoring files with "+" in them seems to be at all possible.

Steps to reproduce the issue:

1. Create a .dockerignore:

/o++o

2. Create a Dockerfile:

FROM alpine:3.11.5
ADD . /src
RUN ls /src

3. Run touch o++o.
4. Run docker build .

Second scenario: do everything the same as above, but use o+o instead of o++o.

Describe the results you received:

First scenario:

error checking context: 'syntax error in pattern'.

Second scenario:

[...]
 ---> Running in a9dfccb86d27
Dockerfile
o+o
[...]

Describe the results you expected:

Both scenarios:

[...]
 ---> Running in a9dfccb86d27
Dockerfile
[...]

Output of docker version:

Client: Docker Engine - Community
 Version:           19.03.8
 API version:       1.40
 Go version:        go1.12.17
 Git commit:        afacb8b7f0
 Built:             Wed Mar 11 01:25:56 2020
 OS/Arch:           linux/amd64
 Experimental:      false

Server: Docker Engine - Community
 Engine:
  Version:          19.03.8
  API version:      1.40 (minimum version 1.12)
  Go version:       go1.12.17
  Git commit:       afacb8b7f0
  Built:            Wed Mar 11 01:24:28 2020
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.2.13
  GitCommit:        7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc:
  Version:          1.0.0-rc10
  GitCommit:        dc9208a3303feef5b3839f4323d9beb36df0a9dd
 docker-init:
  Version:          0.18.0
  GitCommit:        fec3683

Output of docker info:

Client:
 Debug Mode: false

Server:
 Containers: 2
  Running: 0
  Paused: 0
  Stopped: 2
 Images: 35
 Server Version: 19.03.8
 Storage Driver: overlay2
  Backing Filesystem: <unknown>
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  seccomp
   Profile: default
 Kernel Version: 4.9.87-xxxx-std-ipv6-64
 Operating System: Debian GNU/Linux 10 (buster)
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 15.57GiB
 Name: ra
 ID: 2VMD:IG4A:U2Q2:BG7L:FQPZ:OITF:6WOG:M3SR:GQHU:NPFN:7P2C:F3KH
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Username: iphydf
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No cpu cfs quota support
WARNING: No cpu cfs period support

Additional environment details (AWS, VirtualBox, physical, etc.):

Physical, but I doubt it matters.

The issue names no source files or tests; first run the two Docker build reproductions with .dockerignore patterns /o++o and /o+o. Trace the .dockerignore pattern handling that produces the syntax error or fails to ignore o+o, then add regression coverage showing both filenames are excluded from the build context.