Fix the docker_target parameter for toolchains

We have to update the configure parameter to actually use docker_target. Once that is setup, we have to make sure that the executable is added to the runfiles, and update the entrypoint script to find the built binary. This should resolve bazelbuild#2170

.bazelrc

@@ -57,7 +57,9 @@ test:remote --remote_timeout=3600
 test:remote --keep_going
 
 # Configuration specific to buildkite CI testing the default workspace
-test:buildkite --action_env=PATH 
-test:buildkite --define=ENV_KEY=my_key # for tests/container:set_env_make_vars_test 
-test:buildkite --define=ENV_VALUE=my_value # for tests/container:set_env_make_vars_test 
+test:buildkite --action_env=PATH
 test:buildkite --test_output=errors
+
+# A test verifies that these values can be expanded as make vars
+test --define=ENV_KEY=my_key # for tests/container:set_env_make_vars_test
+test --define=ENV_VALUE=my_value # for tests/container:set_env_make_vars_test

container/image.bzl

@@ -550,8 +550,12 @@ def _impl(
         command = "cp %s %s" % (config_digest.path, output_config_digest.path),
     )
 
+    toolchain_info = ctx.toolchains["@io_bazel_rules_docker//toolchains/docker:toolchain_type"].info
+    runfiles_files = [config_file, config_digest, output_config_digest]
+    if toolchain_info.tool_target:
+        runfiles_files.append(toolchain_info.tool_target.files_to_run.executable)
     runfiles = ctx.runfiles(
-        files = unzipped_layers + diff_ids + [config_file, config_digest, output_config_digest] +
+        files = unzipped_layers + diff_ids + runfiles_files +
                 ([container_parts["legacy"]] if container_parts["legacy"] else []),
     )
 

container/incremental_load.sh.tpl

@@ -31,9 +31,8 @@ function guess_runfiles() {
 
 RUNFILES="${PYTHON_RUNFILES:-$(guess_runfiles)}"
 
-DOCKER="%{docker_tool_path}"
+DOCKER="$(readlink -nf %{docker_tool_path})"
 DOCKER_FLAGS="%{docker_flags}"
-
 if [[ -z "${DOCKER}" ]]; then
     echo >&2 "error: docker not found; do you need to manually configure the docker toolchain?"
     exit 1

toolchains/docker/toolchain.bzl

@@ -126,7 +126,7 @@ def _toolchain_configure_impl(repository_ctx):
 
     tool_attr = ""
     if repository_ctx.attr.docker_target:
-        tool_attr = "tool_target = \"%s\"," % repository_ctx.attr.tool_target
+        tool_attr = "tool_target = \"%s\"," % repository_ctx.attr.docker_target
     elif repository_ctx.attr.docker_path:
         tool_attr = "tool_path = \"%s\"," % repository_ctx.attr.docker_path
     elif repository_ctx.which("docker"):