ci: build.yml checks every push and PR; every job on Ubuntu 24.04 with Node (closes #18)

build.yml, as master had it, could not run here: it called
Makefile.starkernel and make T=<board> (the tree has kernel/Makefile
and TARGET=<board>), listed a Pi 4 with no port, published "releases"
under org/f18 on every push, installed Isabelle2024 (the proofs need
Isabelle2025-2) from a mirror that answered 503, and duplicated the
nightlies with its own 02:00 schedule. Now it is the check on every
push to master and StarForth-v4.0.0 and every PR into master: each
board's image (make boot_image TARGET=PC|RASPI|MILKV, xz'd, kept with
its SBOM) and make -C v4 test, both cell widths. It publishes nothing;
the nightlies and release.yml do.

Every job of every workflow now names its container,
gitea/runner-images:ubuntu-24.04-v26.11.03: the runner maps
ubuntu-latest to node:20-bullseye, Debian 11, where every job of PR
#30's runs failed at its first apt-get, and which has no
qemu-efi-riscv64 and a far older QEMU and compilers than the Ubuntu
24.04 all of this was tested on. Node is needed in the image:
actions/checkout and upload-artifact run inside the job's container.
The dated tag keeps a run from changing under an image update.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GE3SDvQn5EoxdGufk84zz2

.gitea/workflows/nightly-docs.yml

@@ -18,6 +18,8 @@ jobs:
   docs:
     name: Book, volumes, API reference
     runs-on: ubuntu-latest
+    container:
+      image: gitea/runner-images:ubuntu-24.04-v26.11.03   # the runner's own ubuntu-latest is Debian 11 (node:20-bullseye)
     timeout-minutes: 90
     steps:
       - uses: actions/checkout@v4

.gitea/workflows/nightly.yml

@@ -20,6 +20,8 @@ jobs:
   nightly:
     name: Acceptance, proofs, images
     runs-on: ubuntu-latest
+    container:
+      image: gitea/runner-images:ubuntu-24.04-v26.11.03   # the runner's own ubuntu-latest is Debian 11 (node:20-bullseye)
     timeout-minutes: 150
     steps:
       - uses: actions/checkout@v4

.gitea/workflows/release.yml

@@ -17,6 +17,8 @@ jobs:
   release:
     name: Release ${{ github.ref_name }}
     runs-on: ubuntu-latest
+    container:
+      image: gitea/runner-images:ubuntu-24.04-v26.11.03   # the runner's own ubuntu-latest is Debian 11 (node:20-bullseye)
     timeout-minutes: 240
     steps:
       - uses: actions/checkout@v4