fix(installer): accept a TestGen image that is already local

[rboni-dk]

tg install --image refused any image built locally:

The Docker engine could not access TestGen's image.

The requirement ran docker manifest inspect, which asks the registry. That answers "is this image published?", but what the install needs to know is "can Docker obtain this image?" — and one already in the local engine needs no pull at all. So a locally built image could never satisfy the check, which makes testing a dev build of TestGen impossible.

Requirement gains an optional alt_cmd, tried when cmd fails, and the image check now asks the local engine first and falls back to the registry. Local first because it is instant, and because it lets an install proceed from cache when the registry is unreachable.

This keeps the diagnostic the check was added for: a name that is neither local nor pullable still fails with the same message. An opt-out flag would not have — it disables the check wholesale, so a user hitting a genuine registry problem and reaching for it would lose exactly the signal they need.

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
dk-installer.py	1820	427	77%	155–156, 160–161, 173–185, 213–214, 217, 222–226, 236–237, 246–258, 263, 276, 322–323, 382, 422–429, 436, 448, 470–471, 520, 570–573, 592–593, 595, 645–674, 678–715, 746–764, 835, 906–907, 975, 992–993, 1010–1015, 1089–1090, 1100, 1116–1120, 1123, 1126, 1129, 1140–1173, 1282, 1288, 1334, 1337, 1344–1345, 1351–1353, 1453–1464, 1513–1528, 1531–1542, 1559–1560, 1597–1598, 1624–1625, 1632, 1637, 1643–1644, 1677–1680, 1689–1690, 1695–1696, 1712–1723, 1769–1771, 1981–2020, 2061–2064, 2129–2132, 2185, 2210–2212, 2220–2222, 2271, 2273, 2275, 2387–2388, 2406, 2436, 2445, 2448–2450, 2493–2505, 2510–2517, 2525–2535, 2596–2609, 2614–2615, 2633–2634, 2674–2675, 2689, 2735, 2847–2917, 2920–2922, 2925–2927, 2998, 3041–3059, 3063, 3068, 3120–3122, 3125–3127, 3153–3156, 3175–3191, 3194–3196, 3199–3201, 3225–3230, 3246–3248, 3284–3292, 3295–3297, 3305–3309, 3373–3375, 3382–3384, 3395–3398, 3403–3404, 3410–3411, 3422–3431, 3440–3494, 3498–3534, 3538–3553
TOTAL	1820	427	77%

Tests	Skipped	Failures	Errors	Time
193	0 💤	0 ❌	0 🔥	2.407s ⏱️