refactor: add initial support for builder plugins

A builder plugin implements some method for 'building' artifacts from a given source code archive. This refactor moves the majority of the image building code into separate plugins, allowing us to implement other builders, but also simplifying the common pugin codebase.

Also move the pre-build-buildpack hook from the build-env plugin to builder-herokuish.

plugins/builder-dockerfile/builder-build

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+source "$PLUGIN_AVAILABLE_PATH/config/functions"
+
+trigger-builder-build-builder-dockerfile() {
+  declare desc="builder-dockerfile builder-build plugin trigger"
+  declare trigger="trigger-builder-build-builder-dockerfile"
+  declare BUILDER_TYPE="$1" APP="$2" SOURCECODE_WORK_DIR="$3"
+
+  if [[ "$BUILDER_TYPE" != "dockerfile" ]]; then
+    return
+  fi
+
+  local IMAGE=$(get_app_image_name "$APP")
+  local DOCKER_BUILD_LABEL_ARGS="--label=org.label-schema.schema-version=1.0 --label=org.label-schema.vendor=dokku --label=com.dokku.app-name=$APP"
+
+  pushd "$SOURCECODE_WORK_DIR" &>/dev/null
+
+  # extract first port from Dockerfile
+  local DOCKERFILE_PORTS=$(get_dockerfile_exposed_ports Dockerfile)
+  [[ -n "$DOCKERFILE_PORTS" ]] && config_set --no-restart "$APP" DOKKU_DOCKERFILE_PORTS="$DOCKERFILE_PORTS"
+
+  # extract ENTRYPOINT/CMD from Dockerfile
+  local DOCKERFILE_ENTRYPOINT=$(extract_directive_from_dockerfile Dockerfile ENTRYPOINT)
+  [[ -n "$DOCKERFILE_ENTRYPOINT" ]] && config_set --no-restart "$APP" DOKKU_DOCKERFILE_ENTRYPOINT="$DOCKERFILE_ENTRYPOINT"
+  local DOCKERFILE_CMD=$(extract_directive_from_dockerfile Dockerfile CMD)
+  [[ -n "$DOCKERFILE_CMD" ]] && config_set --no-restart "$APP" DOKKU_DOCKERFILE_CMD="$DOCKERFILE_CMD"
+  plugn trigger pre-build-dockerfile "$APP"
+
+  [[ "$DOKKU_DOCKERFILE_CACHE_BUILD" == "false" ]] && DOKKU_DOCKER_BUILD_OPTS="$DOKKU_DOCKER_BUILD_OPTS --no-cache"
+  local DOCKER_ARGS=$(: | plugn trigger docker-args-build "$APP" "$BUILDER_TYPE")
+  DOCKER_ARGS+=$(: | plugn trigger docker-args-process-build "$APP" "$IMAGE_TAG" "$BUILDER_TYPE")
+
+  # strip --volume and -v args from DOCKER_ARGS
+  local DOCKER_ARGS=$(sed -e "s/--volume=[[:graph:]]\+[[:blank:]]\?//g" -e "s/-v[[:blank:]]\?[[:graph:]]\+[[:blank:]]\?//g" <<<"$DOCKER_ARGS")
+  declare -a ARG_ARRAY
+  eval "ARG_ARRAY=($DOCKER_ARGS)"
+
+  "$DOCKER_BIN" build "${DOCKER_BUILD_LABEL_ARGS[@]}" $DOKKU_GLOBAL_BUILD_ARGS "${ARG_ARRAY[@]}" "${DOKKU_DOCKER_BUILD_OPTS[@]}" -t $IMAGE .
+
+  plugn trigger post-build-dockerfile "$APP"
+}
+
+trigger-builder-build-builder-dockerfile "$@"

plugins/builder-dockerfile/builder-release

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+set -eo pipefail
+[[ $DOKKU_TRACE ]] && set -x
+
+trigger-builder-release-builder-dockerfile() {
+  declare desc="builder-dockerfile builder-release plugin trigger"
+  declare trigger="trigger-builder-release-builder-dockerfile"
+  declare BUILDER_TYPE="$1" APP="$2" IMAGE_TAG="$3"
+
+  if [[ "$BUILDER_TYPE" != "herokuish" ]]; then
+    return
+  fi
+
+  # buildstep plugins don't necessarily make sense for dockerfiles. call the new breed!!!
+  plugn trigger pre-release-dockerfile "$APP" "$IMAGE_TAG"
+  plugn trigger post-release-dockerfile "$APP" "$IMAGE_TAG"
+}
+
+trigger-builder-release-builder-dockerfile "$@"

plugins/builder-dockerfile/plugin.toml

@@ -0,0 +1,4 @@
+[plugin]
+description = "dokku core builder-dockerfile plugin"
+version = "0.17.9"
+[plugin.config]