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

go.mod

@@ -16,6 +16,7 @@ require (
 	github.com/stretchr/testify v1.8.4
 	github.com/tyler-smith/go-bip39 v1.1.0
 	golang.org/x/crypto v0.38.0
+	golang.org/x/sys v0.33.0
 	golang.org/x/term v0.32.0
 )
 
@@ -31,7 +32,6 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/spf13/pflag v1.0.6 // indirect
-	golang.org/x/sys v0.33.0 // indirect
 	golang.org/x/text v0.25.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
 )