Stamp the tag or short commit in a plain docker build (closes #211)

A plain `docker build .` stamped `dev`: `.dockerignore` left out `.git`
and the Dockerfile defaulted VERSION to `dev`. `.dockerignore` is now
the canonical one plus this repo's entries, sending `.git` without
`.git/config`. Given no build arguments, the builder stamps `git
describe --tags --always` and the commit and date from git, and fails
if `.git` is present but yields no version. The empty CHECK_EPOCH
refusal is gone so the plain build succeeds; the scripts still pass an
epoch. `script/version` now prints `git describe --tags --always
--dirty`, so make, the scripts and a plain build agree.

Model: opus-5-5

.goreleaser.yaml

@@ -47,9 +47,7 @@ checksum:
 # A snapshot is not a release and must not name itself like one. The
 # previous `{{ incpatch .Version }}-next` derived a plausible-looking
 # release number from the last tag -- and with no tags in the repo at
-# all, from goreleaser's fabricated v0.0.0. This produces the same
-# string script/version produces for an untagged build, so a snapshot
-# binary and a `make vaultik` binary of the same clean commit agree.
+# all, from goreleaser's fabricated v0.0.0.
 snapshot:
   version_template: "dev-{{ slice .FullCommit 0 12 }}"