Add dockerfile for archives

What does this MR do and why?

Adds a single.dockerfile to be used when we build the archives.

For #35 (closed).

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.

2. Test building the Docker image:

   # Set environment variables
   export HUGO_VERSION="0.139.4"
   export COREPACK_VERSION="0.30.0"
   export CI_COMMIT_REF_NAME="17.6"
   export IMAGE_NAME="docs:17.6"
   
   # Run the docker build command
   docker build \
    --build-arg HUGO_VERSION=${HUGO_VERSION} \
    --build-arg COREPACK_VERSION=${COREPACK_VERSION} \
    --build-arg CI_COMMIT_REF_NAME=${CI_COMMIT_REF_NAME} \
    --tag ${IMAGE_NAME} \
    --file dockerfiles/single.Dockerfile \
    .

3. Check that everything works (versioned docs, search, etc.):

   docker run -it --rm -p 4000:4000 docs:17.6