kokoro: Avoid docker build from stdin to support COPY

Under the old command, the build would fail with:

    Step 15/25 : COPY pkgconfig/* /usr/lib/pkgconfig
    COPY failed: no source files were specified

The Docker documentation at
https://docs.docker.com/v18.03/engine/reference/builder/
explains:

    Note: If you build using STDIN (docker build - < somefile), there is
    no build context, so COPY cant be used.

TEST=it built successfully

Change-Id: I4d9fcb085009f4a47d7caecabf6eb711f5cab6b7
Reviewed-on: https://chromium-review.googlesource.com/1431356
Commit-Ready: David Tolnay <dtolnay@chromium.org>
Tested-by: David Tolnay <dtolnay@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/kokoro/README.md b/kokoro/README.md
index 48a23a2..201ab54 100644
--- a/kokoro/README.md
+++ b/kokoro/README.md
@@ -12,8 +12,7 @@ is built with a [`Dockerfile`](Dockerfile).
 Assuming a Docker daemon is already running, build the `crosvm-base` image:
 
 ```shell
-cd crosvm/kokoro
-docker build -t crosvm-base - < Dockerfile
+docker build -t crosvm-base path/to/crosvm/kokoro
 ```
 
 Here is how to use the image to test a crosvm repository located at `$CROSVM_SRC`:
@@ -33,8 +32,7 @@ If an update or new library is needed or any other adjustment is required, a new
 generated as follows:
 
 ```shell
-cd crosvm/kokoro
-docker build -t crosvm-base - < Dockerfile
+docker build -t crosvm-base path/to/crosvm/kokoro
 docker save crosvm-base | xz -T 0 -z >crosvm-base.tar.xz
 ```