Dynamically resolve Docker build args (follow bosh pattern)

The ci/docker/openstack-cpi-release/Dockerfile currently pins the BOSH CLI version via a hardcoded ENV BOSH_CLI_VERSION variable. Bumping the version requires a Dockerfile commit.

bosh solves this more cleanly:

1. A dedicated build-docker-args task (ci/dockerfiles/integration/build-docker-args.sh) queries the GitHub API for the latest release URLs of all required tools and writes them to docker-build-args/docker-build-args.yml.
2. The oci-build-task step consumes this file via BUILD_ARGS_FILE: docker-build-args/docker-build-args.yml.
3. The Dockerfile receives tool URLs via ARG instead of hardcoded values - version bumps happen automatically without any committed change.

We should adopt the same approach:

• Replace ENV BOSH_CLI_VERSION with ARG BOSH_CLI_URL in the Dockerfile.
• Add a build-docker-args task + shell script that resolves the latest BOSH CLI release URL from the GitHub API.
• Wire it into the create-openstack-cpi-release-docker-image pipeline job before the build-image step.

The same applies to the bosh-google-cpi-release ci/docker/bosh-google-cpi-boshrelease/Dockerfile, which has ENV BOSH_CLI_VERSION=7.10.5 with the same hardcoded-version pattern.

Start with ci/docker/openstack-cpi-release/Dockerfile and the create-openstack-cpi-release-docker-image pipeline job. Compare the linked BOSH ci/dockerfiles/integration/build-docker-args.sh and Dockerfile to understand the expected build-args flow, then account for the analogous bosh-google-cpi-release Dockerfile. Done means both CPI image builds resolve the latest BOSH CLI URL through the pipeline instead of a committed version.