executors: deploy executors with compose (#901)

* add executors to compose deployment

Author: davejrt

docker-compose/executors/README.md

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+# Executors
+
+Executors are Sourcegraph’s solution for running untrusted code in a secure and controllable way. For more information on executors and how they are used see the Executors [documentation](https://docs.sourcegraph.com/admin/executors)
+
+## Deploying
+
+This directory contains a compose file to deploy a Sourcegraph Executor.
+
+NOTE: Executors require privileged access in order to run correctly on docker-compose based deployments.
+
+To learn more visit TODO
+
+To run this as part of a Sourcegraph deployment on the machine machine, execute the following command from the `docker-compose` directory:
+
+```bash
+docker-compose -f docker-compose.yaml -f executors/executor.docker-compose.yaml up -d
+```
+
+To run this on a standalone machine, execute the following from the `executors` directory
+
+```bash
+docker-compose up -d executor.docker-compose.yaml
+```

docker-compose/executors/executor.docker-compose.yaml

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+version: '2.4'
+services:
+  executor:
+    container_name: executor
+    image: 'index.docker.io/sourcegraph/executor:insiders@sha256:21ed2073df3ae60ac548546e6ecc3a2c3c00b31c635df67508ee11079d5bf156'
+    cpus: 1
+    mem_limit: '4g'
+    # Run as root (required for docker daemon control)
+    user: root
+    # Run with privileged capabilities (required for docker daemon control)
+    privileged: true
+    environment:
+      # Refer to https://docs.sourcegraph.com/admin/deploy_executors_binary#step-2-setup-environment-variables on how to populate these variables
+      - EXECUTOR_FRONTEND_URL=http://sourcegraph-frontend-0:3080
+      # Note: Must match `executors.accessToken` in site config
+      - EXECUTOR_FRONTEND_PASSWORD=
+      # Choose what work to process
+      - EXECUTOR_QUEUE_NAME=
+      # Note: Must match left-hand side of scratch volume mount
+      - EXECUTOR_DOCKER_HOST_MOUNT_PATH=/tmp/sourcegraph/executor-scratch
+      # Note: Must match right-hand side of scratch volume mount
+      - TMPDIR=/scratch
+      # Run as root (required for docker daemon control)
+      - UID=1000
+      - GID=1000
+    volumes:
+      # Mount docker socket
+      - '/var/run/docker.sock:/var/run/docker.sock'
+      # Mount volume for workspaces shared by executor and launched containers
+      - '/tmp/sourcegraph/executor-scratch:/scratch'
+    networks:
+      - sourcegraph
+    restart: always