Add Python 3.14 builds (#361)

[dimitrijepetrovic]

Adds Python 3.14 to the Docker build matrix, producing py3.14-tagged images for both ghcr.io/dask/dask and ghcr.io/dask/dask-notebook.

Changes:

• Add "3.14" to the Python matrix in .github/workflows/build.yml
• Add py3.14 badge entries to README.md

Related to #357 which has failed tests since December 2025.
3.13 was merged in a separate PR

[dimitrijepetrovic]

This part of the run seems to fail intermittently... possibly some kind of rate limiting on nvidia's website... we do hit it with all jobs simultaneously.

It might be worth redoing the CI a bit to execute this first and use the output in all other jobs?

LATEST_RELEASE=$(curl --retry 6 --retry-delay 10 -s https://api.github.com/repos/NVIDIA/container-canary/releases/latest | jq -r ".assets[] | select(.name | test(\"canary_linux_amd64$\")) | .browser_download_url")
  LATEST_RELEASE=$(curl --retry 6 --retry-delay 10 -s https://api.github.com/repos/NVIDIA/container-canary/releases/latest | jq -r ".assets[] | select(.name | test(\"canary_linux_amd64$\")) | .browser_download_url")
  curl --retry 6 --retry-delay 10 -sSL $LATEST_RELEASE > /usr/local/bin/canary
  chmod +x /usr/local/bin/canary
  canary version
  shell: /usr/bin/bash -e {0}
  env:
    release: 2026.3.0
    defaultpython: 3.10
jq: error (at <stdin>:1): Cannot iterate over null (null)

[jrbourbeau]

Thanks @dimitrijepetrovic -- this is in