.dockerignore not considered when computing cache keys

[michaelhixson]

Actual behavior
Files excluded by .dockerignore are wrongly included when determining whether the cache can be used for a COPY build step.

Expected behavior
Modifications to files excluded by .dockerignore do not affect the cacheability of COPY steps.

To Reproduce

Project files

example/project/.dockerignore

files/a.txt

example/project/Dockerfile

FROM alpine
COPY files files
CMD ["ls", "files"]

example/project/files/a.txt

hi

example/project/files/b.txt

bye

example/docker_config.json

{
  "auths": {
    REDACTED
  }
}

example/docker-compose.yml [*]

version: "3.7"
services:
  foo:
    image: gcr.io/kaniko-project/executor:latest
    volumes:
      - ./project:/workspace
      - ./docker_config.json:/kaniko/.docker/config.json
    command:
      - --context=/workspace
      - --destination=REDACTED
      - --cache=true

[*] I would have used docker run ... instead of docker-compose except I couldn't figure out the Windows-compatible syntax that docker run wanted for mounting volumes.

Command line instructions

I do not include the full output of each command. Instead I include my own comments about the output ### formatted like this.

$ cd example

$ docker build project
### OK: docker won't use a cache for the "COPY files files" step
###     since it's the first build

$ docker build project
### OK: this time docker will use a cache for "COPY files files"

$ echo "hi2" >> project/files/a.txt
$ docker build project
### OK: docker will still use a cache for "COPY files files"
###     because files/a.txt was .dockerignored!

$ docker-compose up --build
### OK: kaniko won't use a cache for the "COPY files files" step
###     since it's the first build

$ docker-compose up --build
### OK: this time kaniko will use a cache for "COPY files files"

$ echo "hi3" >> project/files/a.txt
$ docker-compose up --build
### BAD: kaniko won't use a cache for "COPY files files"
###      but it should, since files/a.txt was dockerignored

Additional Information

$ docker images gcr.io/kaniko-project/executor --digests
REPOSITORY                       TAG                 DIGEST                                                                    IMAGE ID            CREATED             SIZE
gcr.io/kaniko-project/executor   latest              sha256:9c40a04cf1bc9d886f7f000e0b7fa5300c31c89e2ad001e97eeeecdce9f07a29   4afd732d4dfa        12 days ago         61.3MB

My theory for the issue's cause
While copy.go's ExecuteCommand takes exclusions from .dockerignore into account, the FilesUsedFromContext function from that same file does not consider exclusions, and that second function is what's used to compute cache keys. Either FilesUsedFromContext or its caller in build.go should be changed to be aware of .dockerignore.

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

[michaelhixson]

Oops, I didn't see #854 before filing this, but I bet that PR will fix this.

[tejal29]

This is closed via #924 and released in kaniko v0.17.0