Handle multiline docker ENV commands correctly

[corneliusweig]

Expected behavior

Skaffold correctly builds dockerfiles with multiline ENV commands.

Actual behavior

Skaffold fails with

FATA[0000] build failed: building [test-build-with-multiline-env]: build artifact: COPY failed: stat <redacted>/tmp/docker-builder563927127/b/file: no such file or directory

Information

• Skaffold version: master
• Operating system: Linux

Steps to reproduce the behavior

1. preparation:

   # prepare folders environment:
   mkdir b && echo "file in dir b" > b/file
   mkdir a && echo "file in dir a" > a/file

2. skaffold.yaml:

   apiVersion: skaffold/v1beta4
   kind: Config
   build:
     artifacts:
     - image: test-build-with-multiline-env

3. Dockerfile:

   FROM busybox
   
   ENV AAA=a \
       BBB=b
   
   COPY ./${BBB}/file /tmp/
   

4. skaffold build fails, whereas docker build . works fine