Add new docker_image language type.

`docker_image` is intended to be a lightweight hook type similar to system /
script which allows one to use an existing docker image which provides a
hook.

pre_commit/languages/all.py

@@ -1,6 +1,7 @@
 from __future__ import unicode_literals
 
 from pre_commit.languages import docker
+from pre_commit.languages import docker_image
 from pre_commit.languages import golang
 from pre_commit.languages import node
 from pre_commit.languages import pcre
@@ -49,6 +50,7 @@ from pre_commit.languages import system
 
 languages = {
     'docker': docker,
+    'docker_image': docker_image,
     'golang': golang,
     'node': node,
     'pcre': pcre,

pre_commit/languages/docker.py

@@ -77,6 +77,16 @@ def install_environment(
         os.mkdir(directory)
 
 
+def docker_cmd():
+    return (
+        'docker', 'run',
+        '--rm',
+        '-u', '{}:{}'.format(os.getuid(), os.getgid()),
+        '-v', '{}:/src:rw'.format(os.getcwd()),
+        '--workdir', '/src',
+    )
+
+
 def run_hook(repo_cmd_runner, hook, file_args):  # pragma: windows no cover
     assert_docker_available()
     # Rebuild the docker image in case it has gone missing, as many people do
@@ -84,16 +94,8 @@ def run_hook(repo_cmd_runner, hook, file_args):  # pragma: windows no cover
     build_docker_image(repo_cmd_runner, pull=False)
 
     hook_cmd = helpers.to_cmd(hook)
-    entry_executable, cmd_rest = hook_cmd[0], hook_cmd[1:]
-
-    cmd = (
-        'docker', 'run',
-        '--rm',
-        '-u', '{}:{}'.format(os.getuid(), os.getgid()),
-        '-v', '{}:/src:rw'.format(os.getcwd()),
-        '--workdir', '/src',
-        '--entrypoint', entry_executable,
-        docker_tag(repo_cmd_runner),
-    ) + cmd_rest
+    entry_exe, cmd_rest = hook_cmd[0], hook_cmd[1:]
 
+    entry_tag = ('--entrypoint', entry_exe, docker_tag(repo_cmd_runner))
+    cmd = docker_cmd() + entry_tag + cmd_rest
     return xargs(cmd, file_args)

pre_commit/languages/docker_image.py

@@ -0,0 +1,19 @@
+from __future__ import absolute_import
+from __future__ import unicode_literals
+
+from pre_commit.languages import helpers
+from pre_commit.languages.docker import assert_docker_available
+from pre_commit.languages.docker import docker_cmd
+from pre_commit.xargs import xargs
+
+
+ENVIRONMENT_DIR = None
+get_default_version = helpers.basic_get_default_version
+healthy = helpers.basic_healthy
+install_environment = helpers.no_install
+
+
+def run_hook(repo_cmd_runner, hook, file_args):  # pragma: windows no cover
+    assert_docker_available()
+    cmd = docker_cmd() + helpers.to_cmd(hook)
+    return xargs(cmd, file_args)

pre_commit/languages/helpers.py

@@ -41,3 +41,7 @@ def basic_get_default_version():
 
 def basic_healthy(repo_cmd_runner, language_version):
     return True
+
+
+def no_install(repo_cmd_runner, version, additional_dependencies):
+    raise AssertionError('This type is not installable')

pre_commit/languages/pcre.py

@@ -10,11 +10,7 @@ ENVIRONMENT_DIR = None
 GREP = 'ggrep' if sys.platform == 'darwin' else 'grep'
 get_default_version = helpers.basic_get_default_version
 healthy = helpers.basic_healthy
-
-
-def install_environment(repo_cmd_runner, version, additional_dependencies):
-    """Installation for pcre type is a noop."""
-    raise AssertionError('Cannot install pcre repo.')
+install_environment = helpers.no_install
 
 
 def run_hook(repo_cmd_runner, hook, file_args):

pre_commit/languages/script.py

@@ -7,11 +7,7 @@ from pre_commit.xargs import xargs
 ENVIRONMENT_DIR = None
 get_default_version = helpers.basic_get_default_version
 healthy = helpers.basic_healthy
-
-
-def install_environment(repo_cmd_runner, version, additional_dependencies):
-    """Installation for script type is a noop."""
-    raise AssertionError('Cannot install script repo.')
+install_environment = helpers.no_install
 
 
 def run_hook(repo_cmd_runner, hook, file_args):

pre_commit/languages/system.py

@@ -7,11 +7,7 @@ from pre_commit.xargs import xargs
 ENVIRONMENT_DIR = None
 get_default_version = helpers.basic_get_default_version
 healthy = helpers.basic_healthy
-
-
-def install_environment(repo_cmd_runner, version, additional_dependencies):
-    """Installation for system type is a noop."""
-    raise AssertionError('Cannot install system repo.')
+install_environment = helpers.no_install
 
 
 def run_hook(repo_cmd_runner, hook, file_args):