maint(developer): use docker buildx to build docker images

This change verifies that `docker buildx` is available and then uses that
for building the images. For that we apparently need the magic line at
the top of the `Dockerfile`s and set the environment variable
`DOCKER_BUILDKIT`. This solves a problem building docker images on
Linux with an older Docker version where the heredocs didn't work.

resources/docker-images/android/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 
 ARG BASE_VERSION=default

resources/docker-images/base/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 
 ARG DISTRO=ubuntu

resources/docker-images/build.sh

@@ -46,6 +46,7 @@ build_action() {
 
   # shellcheck disable=SC2164
   cd "${platform}"
+  export DOCKER_BUILDKIT=1
   # shellcheck disable=SC2248,SC2086
   docker build ${OPTION_NO_CACHE:-} -t "keymanapp/keyman-${platform}-ci:${build_version}" "${build_args[@]}" .
   # If the user didn't specify particular versions we will additionaly create an image
@@ -67,6 +68,7 @@ test_action() {
     ":${platform}" -- ./build.sh configure,build,test:"${platform}"
 }
 
+check_buildx_available
 check_for_default_values
 convert_parameters_to_args
 

resources/docker-images/core/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 #
 # ARGS used in this file:

resources/docker-images/developer/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 #
 # ARGS used in this file:

resources/docker-images/docker-build.inc.sh

@@ -59,3 +59,15 @@ check_for_default_values() {
 is_default_values() {
   ${is_default_values}
 }
+
+# Check that `docker buildx` is available
+check_buildx_available() {
+  if ! docker --help | grep -q buildx; then
+    if [[ "${OSTYPE:-}" == "linux-gnu" ]]; then
+      builder_echo error "Docker buildx is not available. Please install Docker buildx to use this script. e.g. 'sudo apt install docker-buildx'"
+    else
+      builder_echo error "Docker buildx is not available. Please install Docker buildx to use this script."
+    fi
+    exit 1
+  fi
+}

resources/docker-images/linux/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 
 ARG BASE_VERSION=default

resources/docker-images/web/Dockerfile

@@ -1,3 +1,4 @@
+# syntax=docker/dockerfile:1.4
 # Keyman is copyright (C) SIL Global. MIT License.
 #
 # ARGS used in this file: