Append instead of replace file contents

I think the `DOCKER_OPTS` should be appended to `/etc/default/docker` and not replace the entire contents.
The default name of the bridge created is called 'docker0'.
docs: shrink images

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
docs: change kernel version requirement

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Adding Exec method to native execdriver.
Modified Attach() method to support docker exec.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding support for docker exec in daemon.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding 'exec' command to remote API and CLI.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Import nsenter in docker.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Fix bug in attach handling for docker exec. Add docs for 'docker exec' feature.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Update cli.md to include 'docker exec' feature.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Adding integration tests for docker exec feature.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Address review comments.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Removing 'exec' feature from the CLI until the docker daemon supports resizing of
tty sessions for exec'ed commands.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
tarsum: include xattr headers in to the checksum

this is to enhance the tarsum algorithm, but _MUST_ be done in lock step
with the same for docker-registry. (PR will be cited)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
tarsum: version the addition of xattrs

Now that TarSum can be versioned, move the addition of xattr headers in
to the next version of TarSum

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Merge pull request #8023 from nerab/patch-1

Append instead of replace file contents
graph: validate tags properly & add unit tests

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #8049 from unclejack/fix_tag_validation

graph: validate tags properly & add unit tests
Update integration-cli tests to allow one-letter tag as valid.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8051 from jfrazelle/image-tag-can-be-one-letter-test-update

Update integration-cli tests to allow one-letter tag as valid