Apply dockefile exclude only for first stage

[tejal29]

Fixes #595

Description

Only apply dockerignore file for the first stage.
Add integration test to make sure this test case isnt broken in future.

[MajorBreakfast]

FYI I think this PR implements it exactly like Docker: Docker seems to also respect the dockerignore only on the host filesystem. Inside intermediate container file systems it seems to treat the dockerignore like any other file. Or in other words: When copying form an intermediate container, no .dockerignore applies (neither the file on the host file system, which this PR fixes, nor one on the container file system)

How I checked this

Create file a.txt file and .dockerignore that ignores a.txt, then run this dockerfile

FROM alpine:latest AS intermediate

COPY . /
RUN ls -a # shows .dockerignore, no a.txt
RUN touch a.txt
RUN ls -a # shows .dockerignore and a.txt

FROM alpine:latest

COPY --from=intermediate . /
RUN ls -a # shows .dockerignore and a.txt