Fail the image build when the Nexus token is missing

`docker compose build` without NEXUS_TOKEN produced a Nexus 401 forty
seconds in, worded as a credentials problem when it was really an unset
variable. The mount's `required=true` asserts the secret was *declared*,
not that it holds anything, so an empty one sailed past it and npm sent
an empty `_authToken`.

The install moves to tools/docker-install-deps.sh, which checks the
secret before touching the network and fails in under a second with the
three ways to supply it. It is a file rather than inline shell because
BuildKit echoes an inline command twice on failure and buries the
message.

Also ignore .env. It carries NEXUS_TOKEN, compose reads it at build
time, and it was not gitignored — .env.example stays tracked. Nothing
had been committed: the only prior commit predates all of this.

Verified: empty token fails in 0.2s with the message; .env and the
environment variable both build; the image carries no trace of the
token and serves the full page.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.dockerignore

@@ -11,6 +11,9 @@ tmp
 .claude
 docs
 landing-e2e
+
+# tools/ IS needed: the build stage copies docker-install-deps.sh from it.
+# The mock API servers in there are dev-only but too small to be worth excluding.
 **/*.spec.ts
 test-output
 .env