Purge in the right step

Author: ldionne

docker/lnt.dockerfile

@@ -29,13 +29,14 @@ FROM python:3.10-alpine AS llvm-lnt
 # Install dependencies
 RUN apk update \
   && apk add --no-cache --virtual .build-deps git g++ postgresql-dev yaml-dev \
-  && apk add --no-cache libpq
+  && apk add --no-cache libpq \
+  && apk --purge del .build-deps
 
 # Install LNT itself, without leaving behind any sources inside the image.
 RUN --mount=type=bind,source=.,target=./lnt-source \
     cp -R lnt-source /tmp/lnt-src && \
     cd /tmp/lnt-src && \
-    pip3 install -r requirements.server.txt && apk --purge del .build-deps && \
+    pip3 install -r requirements.server.txt && \
     rm -rf /tmp/lnt-src