BLD: `Dockerfile.base.ubuntu.18.04` build currently fails

Describe the bug

The service that Boost uses to serve up its releases, Bintray.com is retired by JFrog. Boost currently uses JFrog.Arifactory to serve build artifacts.

To Reproduce

Steps to reproduce the behavior:

1. Navigate to Examples/docker/ directory.
2. Push: docker build -f Dockerfile.base.ubuntu.18.04 --build-arg NUM_CPU=4 -t moja/baseimage:ubuntu-18.04 .
3. Check the error:

[+] Building 5.8s (11/33)                                                       
 => [internal] load build definition from Dockerfile.base.ubuntu.18.04     0.0s
 => => transferring dockerfile: 5.30kB                                     0.0s
 => [internal] load .dockerignore                                          0.0s
 => => transferring context: 2B                                            0.0s
 => [internal] load metadata for docker.io/library/ubuntu:bionic           4.4s
 => [auth] library/ubuntu:pull token for registry-1.docker.io              0.0s
 => CANCELED http://download.osgeo.org/gdal/2.4.0/gdal-2.4.0.tar.gz        1.3s
 => CANCELED https://www.sqlite.org/2019/sqlite-autoconf-3270200.tar.gz    1.3s
 => CANCELED https://github.com/fmtlib/fmt/archive/5.3.0.tar.gz            1.3s
 => ERROR https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_6  1.3s
 => CANCELED https://pocoproject.org/releases/poco-1.9.0/poco-1.9.0.tar.g  1.3s
 => CANCELED https://github.com/Kitware/CMake/releases/download/v3.14.3/c  1.3s
 => CANCELED [ 1/24] FROM docker.io/library/ubuntu:bionic@sha256:37b7471c  1.3s
 => => resolve docker.io/library/ubuntu:bionic@sha256:37b7471c1945a2a12e5  0.0s
 => => sha256:37b7471c1945a2a12e5a57488ee4e3e216a8369d0b9 1.41kB / 1.41kB  0.0s
 => => sha256:963f8ca4f8dcda72717dfbe53617aa2a705dc8ebf01abe9 529B / 529B  0.0s
 => => sha256:df8edc186894d22d1f425f0a490b3a5c3a2af96f27f 1.48kB / 1.48kB  0.0s
 => => sha256:976e4515cbe3bb05d3eaff6aa830f00c2593211ff7 7.34MB / 23.73MB  1.3s
------
 > https://dl.bintray.com/boostorg/release/1.69.0/source/boost_1_69_0.tar.bz2:
------
failed to load cache key: invalid response status 502

Expected behavior

The Docker build should succeed.

Operating Environment:

• MacBook Pro M1 2021
• macOS Monterey, Docker version 20.10.11, build dea9396

Start in Examples/docker/Dockerfile.base.ubuntu.18.04 and inspect the Boost 1.69.0 download referenced by the failing build. Run the reported docker build command after the change; done means the Ubuntu 18.04 image builds successfully without the Bintray error.