Multistage builds not copying files

[nfvs]

Actual behavior
Files that are generated in a stage do not get copied over in the next stages.

Expected behavior
Files should get copied over to the next stages

To Reproduce
Steps to reproduce the behavior:

1. Build a simple two-stage image

FROM python:3.5 AS builder
RUN pip install virtualenv
RUN virtualenv /venv -p python3

FROM python:3.5-slim
COPY --from=builder /venv /venv

2. Inspecting the resulting image, we notice there is no /venv. Also this output:

INFO[0117] Taking snapshot of full filesystem...        
INFO[0118] COPY --from=builder /venv /venv              
INFO[0119] Taking snapshot of full filesystem...        
INFO[0119] No files were changed, appending empty layer to config. No layer added to image. 

Additional Information

• Runing this in GitLab using gcr.io/kaniko-project/executor:debug (tested sha256:642ef6209052ed2d96bfc20d2edc2803300ce3d858773629683b6d0903624777 and sha256:3c820e1112b24421d103582dbf27e7fca05b4eb335adaadca3d616d3d48689b9)
• Works fine with Docker:

❯ cat Dockerfile2
FROM python:3.5 AS builder
RUN pip install virtualenv
RUN virtualenv /venv -p python3

FROM python:3.5-slim
COPY --from=builder /venv /venv

❯ docker build . -t test -f Dockerfile2
Sending build context to Docker daemon  547.8kB
Step 1/5 : FROM python:3.5 AS builder
...
Successfully built 05bc82648689
Successfully tagged test:latest

❯ docker run -it test:latest ls -als /venv
total 20
4 drwxr-xr-x 5 root root 4096 Mar 13 16:02 .
4 drwxr-xr-x 1 root root 4096 Mar 13 16:03 ..
4 drwxr-xr-x 2 root root 4096 Mar 13 15:29 bin
4 drwxr-xr-x 2 root root 4096 Mar 13 15:29 include
4 drwxr-xr-x 3 root root 4096 Mar 13 15:29 lib

[nfvs]

Seems like it's an issue with .dockerignore:

DEBU[0060] /kaniko/0/venv found in .dockerignore, ignoring

I have a pretty aggressive .dockerignore that blacklists everything then whitelists individual files, which obviously doesn't include files generated in a previous stage. Maybe a copy-from-previous-stage should ignore the .dockerignore, since we usually don't want to send a local environment as context but may want to generate one and move it between stages?

[viceice]

Duplicate of #595

[dlorenc]

Closing as a dup of #595