feat: export environment variables during dockerfile builds for use with value-less --build-arg flags

Exposing all config values will allow users to skip setting environment variables twice - once as a docker option and once as an env var. Docker will automatically pull the value from the environment if none is set for the --build-arg flag.

Users will still be required to specify each desired env var via --build-arg as otherwise docker builds will complain about unused build arguments.

Re-implements #5978 (lost in a rebase)
Closes #5903

plugins/builder-dockerfile/builder-build

@@ -38,6 +38,7 @@ trigger-builder-dockerfile-builder-build() {
   declare -a ARG_ARRAY
   eval "ARG_ARRAY=($DOCKER_ARGS)"
 
+  eval "$(config_export app "$APP")"
   "$DOCKER_BIN" image build "${DOCKER_BUILD_LABEL_ARGS[@]}" $DOKKU_GLOBAL_BUILD_ARGS "${ARG_ARRAY[@]}" ${DOKKU_DOCKER_BUILD_OPTS} -t $IMAGE .
 
   plugn trigger post-build-dockerfile "$APP"