update versions and improve script

[chrisghill]

Summary

Updates the provisioner's toolchain versions, hardens the Docker image, and
makes a number of robustness/correctness improvements to entrypoint.sh.

Dockerfile / image

• Bump tool versions: Azure CLI 2.75.0 → 2.86.0, Checkov 3.2.268 → 3.2.530.
• Drop OPA (0.69.0) entirely — it was installed but unused.
• Base image: debian:12.7-slim → debian:13.5-slim (and libicu72 → libicu76
  to match).
• Fix latent ARG bug: the build referenced $AZ_CLI_VERSION while the ARG was
  AZURE_CLI_VERSION, so the else branch never pinned a version. Now consistent.
• Harden apt usage: apt-get … --no-install-recommends, add ca-certificates,
  and switch the Microsoft repo to the signed-by=/usr/share/keyrings/microsoft.gpg
  pattern (no more deprecated trusted.gpg.d).
• Copy only what we ship: explicit COPY of xo and checkov instead of a
  /usr/local/bin/* wildcard (OPA is gone).
• User creation: adduser → useradd with a home dir and /bin/bash shell.
• Pre-install Bicep (az bicep install) at build time so az stack doesn't
  download it on first run.

CI

• docker_build_push.yaml: matrix azure_cli_version 2.81.0 → 2.86.0 to match
  the Dockerfile default.

entrypoint.sh

• az flags built as arrays instead of space-joined strings, so values
  containing spaces can't be split into multiple arguments. Removed the unused
  show_flags variable.
• artifact_*.jq and resource_*.jq are both matched (via nullglob +
  continue, with the field name stripped from either prefix).
• Non-fatal artifact/resource deletion on decommission: a failed delete logs a
  warning and continues rather than aborting the teardown.
• Surface deployment failures: stack-create stdout goes to a file; on failure
  it's printed and the script exits non-zero.
• Tighten az group exists to a direct string compare instead of grep -q.