Let a plain docker build pass and stamp the git version (closes #57)

The size tests skip a case whose secret needs more locked memory than
RLIMIT_MEMLOCK allows: memguard panics otherwise, and a plain
`docker build .` runs under an 8 MiB limit. script/cibuild still runs
every case.

The build stage stamps the VERSION build argument, else
`git describe --tags --always`, and fails when .git is present but
yields no version. `make build` stamps `git describe` too instead of
the fixed 0.1.0. .dockerignore keeps .git/config out; script/docker is
now the canonical copy.

Model: opus-5-5

script/cibuild

@@ -1,8 +1,9 @@
 #!/bin/sh
 # script/cibuild: run the CI build. The Dockerfile runs script/check
 # (via make check), so a successful build implies all checks pass.
-# The Gitea workflow runs this on push. The memlock ulimit is required
-# because the test suite uses memguard, which mlocks memory.
+# The Gitea workflow runs this on push. The memlock ulimit lets the tests
+# that lock large secrets in memory (memguard mlocks them) run; under the
+# lower limit of a plain `docker build .` they are skipped.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"