Adding docker_files module proposal

docs/proposals/docker/docker_files_module.md

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+# Docker_Files Modules Proposal
+
+## Purpose and Scope
+
+The purpose of docker_files is to provide for retrieving a file or folder from a container's file system, 
+inserting a file or folder into a container, exporting a container's entire filesystem as a tar archive, or 
+retrieving a list of changed files from a container's file system.
+
+Docker_files will manage a container using docker-py to communicate with either a local or remote API. It will
+support API versions >= 1.14. API connection details will be handled externally in a shared utility module similar to
+how other cloud modules operate.
+
+## Parameters
+
+Docker_files accepts the parameters listed below. API connection parameters will be part of a shared utility module
+as mentioned above.
+
+```
+diff:
+  description:
+    - Provide a list of container names or IDs. For each container a list of changed files and directories found on the
+      container's file system will be returned. Diff is mutually exclusive from all other options except event_type. 
+      Use event_type to choose which events to include in the output.
+  default: null
+
+export:
+  description: 
+    - Provide a container name or ID. The container's file system will be exported to a tar archive. Use dest
+      to provide a path for the archive on the local file system. If the output file already exists, it will not be
+      overwritten. Use the force option to overwrite an existing archive.
+  default: null
+  
+dest:
+  description:
+    - Destination path of copied files. If the destination is a container file system, precede the path with a
+      container name or ID + ':'. For example, C(mycontainer:/path/to/file.txt). If the destination path does not
+      exist, it will be created. If the destination path exists on a the local filesystem, it will not be overwritten.
+      Use the force option to overwrite existing files on the local filesystem.
+  default: null
+
+force: 
+  description:
+    - Overwrite existing files on the local filesystem. 
+  default: false
+  
+follow_link:
+  description:
+    - Follow symbolic links in the src path. If src is local and file is a symbolic link, the symbolic link, not the 
+    target is copied by default. To copy the link target and not the link, set follow_link to true.
+  default: false
+
+event_type:
+  description:
+    - Select the specific event type to list in the diff output.
+  choices:
+    - all
+    - add
+    - delete
+    - change
+  default: all
+
+src:
+  description:
+    - The source path of file(s) to be copied. If source files are found on the container's file system, precede the
+      path with the container name or ID + ':'. For example, C(mycontainer:/path/to/files).
+  default: null
+
+```
+
+## Examples
+
+```
+- name: Copy files from the local file system to a container's file system
+  docker_files:
+    src: /tmp/rpm
+    dest: mycontainer:/tmp
+    follow_links: yes
+
+- name: Copy files from the container to the local filesystem and overwrite existing files
+  docker_files:
+    src: container1:/var/lib/data
+    dest: /tmp/container1/data
+    force: yes
+    
+- name: Export container filesystem
+  docker_file:
+    export: container1
+    dest: /tmp/conainer1.tar
+    force: yes
+    
+- name: List all differences for multiple containers.
+  docker_files:
+    diff:
+      - mycontainer1
+      - mycontainer2
+
+- name: Included changed files only in diff output
+  docker_files:
+    diff:
+      - mycontainer1
+    event_type: change
+```
+
+## Returns
+
+Returned from diff:
+
+```
+{
+    changed: false,
+    failed: false,
+    rc: 0,
+    results: {
+        mycontainer1: [
+            { state: 'C', path: '/dev' },
+            { state: 'A', path: '/dev/kmsg' },
+            { state: 'C', path: '/etc' },
+            { state: 'A', path: '/etc/mtab' }
+        ],
+        mycontainer2: [
+            { state: 'C', path: '/foo' },
+            { state: 'A', path: '/foo/bar.txt' }
+        ]
+    }
+}
+```
+
+Returned when copying files:
+
+```
+{
+    changed: true,
+    failed: false,
+    rc: 0,
+    results: {
+        src: /tmp/rpms,
+        dest: mycontainer:/tmp
+        files_copied: [
+            'file1.txt',
+            'file2.jpg'
+        ]
+    }
+}
+```
+
+Return when exporting container filesystem:
+
+```
+{
+   changed: true,
+    failed: false,
+    rc: 0,
+    results: {
+        src: container_name,
+        dest: local/path/archive_name.tar
+    }
+}
+
+```