Fix contact form POST routing and bake env into build

- nginx: route POST /contact/ to contact.php, GET to router.php
- Dockerfile: COPY .env into image
- entrypoint.sh: source .env on container start
- .dockerignore: allow .env to be included in build context

infra/entrypoint.sh

@@ -1,5 +1,8 @@
 #!/bin/sh
 set -e
+if [ -f /var/www/html/.env ]; then
+    set -a; . /var/www/html/.env; set +a
+fi
 if [ -z "$ALTCHA_HMAC_KEY" ]; then
     export ALTCHA_HMAC_KEY="$(openssl rand -hex 32)"
     echo "Generated ALTCHA_HMAC_KEY" >&2