add force option on docker tag when available. closes #2425

plugins/common/functions

@@ -222,6 +222,11 @@ is_image_herokuish_based() {
   docker run "$DOKKU_GLOBAL_RUN_ARGS" --entrypoint="/bin/sh" $DOCKER_ARGS "$@" -c "test -f /exec" &> /dev/null
 }
 
+get_docker_version() {
+  CLIENT_VERSION_STRING="$(docker version -f="{{ .Client.Version }}")"
+  echo "$CLIENT_VERSION_STRING"
+}
+
 is_number() {
   declare desc="returns 0 if input is a number"
   local NUMBER=$1; local NUM_RE='^[0-9]+$'

plugins/tags/functions

@@ -2,7 +2,23 @@
 set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
 source "$PLUGIN_CORE_AVAILABLE_PATH/common/functions"
 
+is_tag_force_available() {
+  CLIENT_VERSION_STRING="$(get_docker_version)"
+  MAJOR_VERSION=$(echo "$CLIENT_VERSION_STRING" | awk '{split($0,a,"."); print a[1]}')
+  MINOR_VERSION=$(echo "$CLIENT_VERSION_STRING" | awk '{split($0,a,"."); print a[2]}')
+  PATCH_VERSION=$(echo "$CLIENT_VERSION_STRING" | awk '{split($0,a,"."); print a[3]}')
+
+  # docker tag -f was dropped in 1.12.0
+  if [[ "$MAJOR_VERSION" -ge 1 ]] && [[ "$MINOR_VERSION" -ge 12 ]]; then
+    return 1
+  else
+    return 0
+  fi
+}
+
 tag_image() {
   declare desc="convenience method for docker tag interface"
-  docker tag -f "$1" "$2"
+  [[ $(is_tag_force_available) ]] && TAG_OPTS="-f"
+  # shellcheck disable=SC2086
+  docker tag $TAG_OPTS "$1" "$2"
 }