kolla-build: fix build-date LABEL invalidating Docker layer cache

The build-date label was rendered into a LABEL instruction on line
3 of every Dockerfile template, immediately after FROM.

Because Docker invalidates the cache for a changed instruction and
all subsequent layers, a date change caused a full rebuild of every
image on each new calendar day, even when nothing else had changed.

Fix this by removing build-date from the Dockerfile templates and
applying it as a --label argument at build time via image.labels.

Labels passed on the build command line are not part of the
Dockerfile cache key, so the date is still recorded in the image
metadata without affecting layer caching.

Change-Id: I8ef0d85bab1a999a4d09f7cbf0c088ca00fdcf77
Signed-off-by: Michal Nasiadka <mnasiadka@gmail.com>

2 files changed, 2 insertions, 2 deletions

diff --git a/doc/source/admin/image-building.rst b/doc/source/admin/image-building.rst
index 842198470..af59e4a2e 100644
--- a/doc/source/admin/image-building.rst
+++ b/doc/source/admin/image-building.rst
@@ -615,7 +615,7 @@ First, create a ``Dockerfile.j2`` template for the project.
    FROM {{ namespace }}/{{ image_prefix }}openstack-base:{{ tag }}
 
    {% block labels %}
-   LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build_date }}"
+   LABEL maintainer="{{ maintainer }}" name="{{ image_name }}"
    {% endblock %}
 
    {% block releaser_header %}{% endblock %}
diff --git a/doc/source/contributor/adding-a-new-image.rst b/doc/source/contributor/adding-a-new-image.rst
index 147d77ad9..995bf13c7 100644
--- a/doc/source/contributor/adding-a-new-image.rst
+++ b/doc/source/contributor/adding-a-new-image.rst
@@ -33,7 +33,7 @@ OpenStack project base image is as follows:
 
    FROM {{ namespace }}/{{ image_prefix }}openstack-base:{{ tag }}
    {% block labels %}
-   LABEL maintainer="{{ maintainer }}" name="{{ image_name }}" build-date="{{ build_date }}"
+   LABEL maintainer="{{ maintainer }}" name="{{ image_name }}"
    {% endblock %}
 
    {% block << service >>_header %}{% endblock %}