ACR Build for Dockerfile fails through extension

[sajayantony]

Issue Type: Bug

Repro files located here - https://github.com/sajayantony/dockerfiles/tree/master/arm

The following dependencies were found:
- image:
    registry: sajaywcus.azurecr.io
    repository: curl
    tag: da2p
    digest: sha256:666b7069d7007140de69d256fa58846328cc0cc025fd5afa297a54f6571b955b
  runtime-dependency:
    registry: registry.hub.docker.com
    repository: library/alpine
    tag: latest
    digest: sha256:621c2f39f8133acb8e64023a94dbdf0d5ca81896102b9e57c0dc184cadaf5528
  git: {}

2018/11/26 22:35:25 Successfully populated digests for step ID: build
2018/11/26 22:35:25 Step ID: push marked as successful (elapsed time in seconds: 31.557586)

Run ID: da2p was successful after 49s
Setting up temp file with 'sourceArchive567990.tar.gz'
   Sending source code to temp file
   Getting build source upload URL 
   Getting blob info from upload URL 
   Creating blob service 
   Creating block blob 
Uploaded source code to /var/folders/y0/b2l3cygn1zx6mc86qklppwx80000gn/sourceArchive567990.tar.gz
Set up run request
Scheduled run da2q
2018/11/26 22:36:30 Using acb_vol_b7e1045e-705a-4cd9-9375-48de0b52a35b as the home volume
2018/11/26 22:36:30 Setting up Docker configuration...
2018/11/26 22:36:31 Successfully set up Docker configuration
2018/11/26 22:36:31 Logging in to registry: sajaywcus.azurecr.io
2018/11/26 22:36:33 Successfully logged in
2018/11/26 22:36:33 Executing step ID: build. Working directory: '', Network: ''
2018/11/26 22:36:33 Obtaining source code and scanning for dependencies...
2018/11/26 22:36:35 Output from dependency scanning: Error: unexpected EOF
Error: failed to run step ID: build: failed to scan dependencies: exit status 1

Extension version: 0.4.0
VS Code version: Code 1.29.1 (bc24f98b5f70467bc689abf41cc5550ca637088e, 2018-11-15T19:06:21.742Z)
OS version: Darwin x64 18.2.0

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-7567U CPU @ 3.50GHz (4 x 3500)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: enabled rasterization: enabled video_decode: enabled video_encode: enabled webgl: enabled webgl2: enabled
Load (avg)	3, 3, 4
Memory (System)	16.00GB (1.03GB free)
Process Argv	.
Screen Reader	no
VM	0%

[rosanch]

fixed by #656