ImageBuild is ignoring build errors

[donutheist]

If my Dockerfile has a COPY command where the file is missing, ImageBuild does not report the error.

Here is my Dockerfile:

FROM gcr.io/distroless/base

COPY ap1.repo /etc/yum.repos.d/ap.repo

Here is my test script

If I run the test script with the COPY command but no file, ImageBuild does not report the error and continues. Here is output.

[gdanko@SDGL141bb265b go]$ go run dockerbuild.go 
Tarring => Dockerfile
*** success ***
{0xc0002f6100 linux}

*** ImageBuild output ***
Step 1/2 : FROM gcr.io/distroless/base

But if I go into the planB directory and run docker build I will end up in error.

[gdanko@SDGL141bb265b planB]$ docker build -t planb .
Sending build context to Docker daemon  2.048kB
Step 1/2 : FROM gcr.io/distroless/base
 ---> 1358ac6604c5
Step 2/2 : COPY ap1.repo /etc/yum.repos.d/ap.repo
COPY failed: stat /var/lib/docker/tmp/docker-builder088607045/ap1.repo: no such file or directory

Now, if I touch the missing file, both the script and the manual docker build work correctly.

[gdanko@SDGL141bb265b go]$ touch planB/ap1.repo
[gdanko@SDGL141bb265b go]$ go run dockerbuild.go 
Tarring => Dockerfile
Tarring => ap1.repo
*** success ***
{0xc0002d8380 linux}

*** ImageBuild output ***
Step 1/2 : FROM gcr.io/distroless/base
---> 1358ac6604c5
Step 2/2 : COPY ap1.repo /etc/yum.repos.d/ap.repo
---> 32459dc23e01
---> 32459dc23e01
Successfully built 32459dc23e01
Successfully tagged dummy:latest

And manually

[gdanko@SDGL141bb265b go]$ cd planB
[gdanko@SDGL141bb265b planB]$ docker build -t planb .
Sending build context to Docker daemon   2.56kB
Step 1/2 : FROM gcr.io/distroless/base
---> 1358ac6604c5
Step 2/2 : COPY ap1.repo /etc/yum.repos.d/ap.repo
 ---> Using cache
 ---> 81d57edf0715
Successfully built 81d57edf0715
Successfully tagged planb:latest