[nominatim-web] Use the slim image variant for build

[Bartłomiej Piotrowski <bpiotrowski src gnome org>]

commit a3d050c2d10324e8cf858f42525672096d19db39
Author: Bartłomiej Piotrowski <bpiotrowski gnome org>
Date:   Sun Feb 6 16:38:52 2022 +0100

    Use the slim image variant for build

 Dockerfile | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)
---
diff --git a/Dockerfile b/Dockerfile
index 3aaa7ac..238253c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,12 +1,12 @@
-FROM python:3.10 as builder
+FROM python:3.10-slim as builder
 
 RUN apt-get update && \
     apt-get install -y --no-install-recommends \
         build-essential libpq-dev
 
+ADD requirements.txt /requirements.txt
 RUN python -m venv /venv && \
-    /venv/bin/pip install fastapi unicorn requests redis hiredis psycopg2 && \
-    rm -f /requirements.txt
+    /venv/bin/pip install -r fastapi uvicorn redis hiredis psycopg2 requests
 
 FROM python:3.10-slim