Make single.Dockerfile a standalone Dockerfile

Dockerfile.single depends on 4 Dockerfiles, something that complicates things. The order goes like this:

1. gitlab-docs-base.Dockerfile
2. bootstrap.Dockerfile
3. builder.onbuild.Dockerfile
4. nginx.onbuild.Dockerfile
5. single.Dockerfile

This MR combines all the Dockerfiles needed for the single versions into one Dockerfile.

To test this locally:

Get the new Dockerfile and test with the 14.1 version:

git fetch origin
git checkout -b "ap-dockerfile-single" "origin/ap-dockerfile-single"
git checkout 15.0
git checkout ap-dockerfile-single -- dockerfiles/single.Dockerfile
cp dockerfiles/single.Dockerfile 15.0.Dockerfile

Replace the X.Y values:

ENV VER=15.0
ENV CI_COMMIT_REF_NAME=15.0
ENV BRANCH_EE=15-0-stable-ee
ENV BRANCH_OMNIBUS=15-0-stable
ENV BRANCH_RUNNER=15-0-stable
ENV BRANCH_CHARTS=6-0-stable

Build the image:

docker build -t docs:15.0 -f 15.0.Dockerfile .

Test this in the browser:

docker run -p 4000:4000 docs:15.0

Visit it at http://0.0.0.0:4000.

---

Resolves #1004 (closed)