Merge branch 'lf/enable-buildkit' into 'lf/specify-image-repo'

Activate Docker BuildKit when available for faster image builds

See merge request !3

container/Dockerfile.varnish

@@ -6,7 +6,9 @@ COPY uplex_varnish.repo /etc/yum.repos.d/
 # yum update with --exclude=shadow-utils because the cap_set_file
 # capability is needed to extract the RPM, and that fails in a
 # docker build.
-RUN yum install -y -q epel-release && yum -q makecache -y fast && \
+# Limit the number of file descriptors to work around a bug in older versions of yum.
+RUN ulimit -n 1024 && \
+    yum install -y -q epel-release && yum -q makecache -y fast && \
     yum update -y -q --exclude=shadow-utils && \
     yum install -y -q varnish-6.3.2 && \
     yum install -y -q --nogpgcheck vmod-re2-1.8.0 && \

container/Makefile

@@ -27,6 +27,7 @@
 all: controller varnish haproxy
 
 DOCKER_BUILD_OPTIONS =
+export DOCKER_BUILDKIT = 1
 
 ifeq ($(origin IMAGE_REPO), undefined)
   IMAGE_REPO =