@babolivier showed me the way

Dockerfile

@@ -31,10 +31,7 @@ FROM nginx:alpine
 COPY --from=builder /src/webapp /app
 
 # Insert wasm type into Nginx mime.types file so they load correctly.
-RUN sed '3i\ \ \ \ application/wasm wasm\;' /etc/nginx/mime.types > /tmp/mime.types
-RUN cp /tmp/mime.types /etc/nginx/mime.types
-
-RUN cat /etc/nginx/mime.types
+RUN sed -i '3i\ \ \ \ application/wasm wasm\;' /etc/nginx/mime.types
 
 RUN rm -rf /usr/share/nginx/html \
  && ln -s /app /usr/share/nginx/html