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)
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
integ-cli: fix TestImportDisplay & add FileServer

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Merge pull request #8037 from unclejack/shrink_docs_images

docs: shrink images
Merge pull request #8036 from unclejack/kernel_version_requirement

docs: change kernel version requirement
Help new users if their SELinux is not-upgraded yet.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #8030 from SvenDowideit/add-help-if-selinux-is-out-of-date

Help new users if their SELinux is not-upgraded yet.
Merge pull request #8034 from michaljemala/patch-1

The default name of the bridge created is called 'docker0'.
Consistently use 'sudo docker' in examples

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #8013 from unclejack/fix_testimportdisplay

integ-cli: fix TestImportDisplay & add FileServer
Added missing 't' from the end of the /images/{{id}}/get eg.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #8060 from SvenDowideit/missing-t-in-get-api-eg

Added missing 't' from the end of the /images/{{id}}/get eg.
Merge pull request #8056 from SvenDowideit/use-sudo-docker-everywhere

Consistently use 'sudo docker' in examples