[jaredgalloway]

Summary

The Quay container build (quay.io/hdc-workflows/phippery) has been
failing — the README's container badge shows "building"/no-status. A full
local docker build reproduction found two stacked failures, both
fixed here:

1. Dead ADD cache-bust endpoint (the immediate blocker).
ADD http://date.jsontest.com /etc/builddate now returns HTTP 403;
Docker/Quay treat a non-200 on ADD <url> as a fatal build error, so the
build died at step 2/5 before installing anything. This is the "internal
system error after the ADD command" seen on Quay's manual build. Removed —
Docker already invalidates the cache on content change, so the manual bust
was both broken and unnecessary.

2. Python 3.8 dependency rot (hidden behind #1).
With the ADD line gone, the build reached the pip install step and
failed there: the base image hdc-workflows/ubuntu:20.04 ships Python
3.8, which is below phippery's own requires-python >=3.9. On 3.8 the
POT dependency has no wheel and builds from source, where its
pre-Cython-3 .pyx fails to compile ('ndarray' is not a type identifier, ~40 errors).

Fix: bump the base image to ubuntu:22.04 (Python 3.10.12). This
matches phippery's Python floor and lets POT install a prebuilt
cp310 wheel — no source compile, no Cython crash. Also modernized the
LABEL to the key=value form (Docker deprecated the legacy key value
form); bumpver's Dockerfile pattern is the bare {version}, so 1.3.1 is
still matched.

Test plan

Verified locally end-to-end with docker build --no-cache (the current
main Dockerfile was reproduced failing first, then this version):

• ✓ Image builds green through the real pip install git+…@1.3.1
• ✓ POT-0.9.7 installs as a wheel (was: source build → Cython crash)
• ✓ docker run … python3 -c "import phippery" → 1.3.1
• ✓ docker run … phippery --help → CLI runs
• ✓ docker run … python3 --version → Python 3.10.12 (≥ 3.9 floor)

Merging to main triggers a fresh Quay build, which is the real
end-to-end confirmation.

Note: local repro was on arm64 (Apple Silicon); Quay builds amd64. The
fix is architecture-independent — POT ships cp310 manylinux wheels
for both, so the "Python 3.10 → wheel, not source build" logic holds on
Quay's x86_64 workers.

Deviations from spec

This PR was scoped by a live decision (not a written issue spec). The user
asked to "remove the ADD line and push to main to test." During local
verification I found the second failure (POT/Cython on py3.8) and asked how
deep to fix; the user chose "ADD + modern base image." The LABEL
modernization was a minor, in-scope cleanup folded into the header change I
was already making. No other departures.

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