Restore README

Author: bthomee

docker/ubuntu/README.md

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+## Ubuntu: A Docker image used to build and test rippled
+
+The code in this repository creates a locked-down Ubuntu image for building and
+testing rippled in the GitHub CI pipelines.
+
+Although the images will be built by a CI pipeline in this repository, if
+necessary a maintainer can build them manually by following the instructions
+below.
+
+### Logging into the Docker registry
+
+To be able to push to GitHub a personal access token is needed, see instructions
+[here](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry#authenticating-with-a-personal-access-token-classic).
+In summary, if you do not have a suitable personal access token, generate one
+[here](https://github.com/settings/tokens/new?scopes=write:packages).
+
+```shell
+DOCKER_REGISTRY=ghcr.io
+GITHUB_USER=<your-github-username>
+GITHUB_TOKEN=<your-github-personal-access-token>
+echo ${GITHUB_TOKEN} | \
+docker login ${DOCKER_REGISTRY} -u "${GITHUB_USER}" --password-stdin
+```
+
+### Building and pushing the Docker image
+
+The same Dockerfile can be used to build an image for Ubuntu 22.04 and 24.04 by
+specifying the `UBUNTU_VERSION` build argument. There are additional arguments
+to specify as well, namely `CMAKE_VERSION` and `GCC_VERSION` for the GCC flavor
+and `CMAKE_VERSION` and `CLANG_VERSION` for the Clang flavor.
+
+Run the commands below from the current directory containing the Dockerfile.
+
+#### Building the Docker image for GCC.
+
+Ensure you've run the login command above to authenticate with the Docker
+registry.
+
+```shell
+UBUNTU_VERSION=noble
+GCC_VERSION=14
+DOCKER_IMAGE=xrplf/ci/${UBUNTU_VERSION}:gcc${GCC_VERSION}
+
+DOCKER_BUILDKIT=1 docker build . \
+  --target gcc \
+  --build-arg BUILDKIT_INLINE_CACHE=1 \
+  --build-arg UBUNTU_VERSION=${UBUNTU_VERSION} \
+  --build-arg GCC_VERSION=${GCC_VERSION} \
+  --tag ${DOCKER_REGISTRY}/${DOCKER_IMAGE} \
+  --platform linux/amd64
+
+docker push ${DOCKER_REGISTRY}/${DOCKER_IMAGE}
+```
+
+#### Building the Docker image for Clang.
+
+Ensure you've run the login command above to authenticate with the Docker
+registry.
+
+```shell
+UBUNTU_VERSION=noble
+CLANG_VERSION=18
+DOCKER_IMAGE=xrplf/ci/${UBUNTU_VERSION}:clang${CLANG_VERSION}
+
+DOCKER_BUILDKIT=1 docker build . \
+  --target clang \
+  --build-arg BUILDKIT_INLINE_CACHE=1 \
+  --build-arg UBUNTU_VERSION=${UBUNTU_VERSION} \
+  --build-arg CLANG_VERSION=${CLANG_VERSION} \
+  --tag ${DOCKER_REGISTRY}/${DOCKER_IMAGE} \
+  --platform linux/amd64
+
+docker push ${DOCKER_REGISTRY}/${DOCKER_IMAGE}
+```