docker: use default entrypoint and command where applicable (#2529)

A default entrypoint and command make it just a bit easier to use the images as
there is no longer a need for an explicit entrypoint. The exception is the
server image, which still requires the shell script to be specified.

web/Dockerfile

@@ -38,3 +38,7 @@ COPY --from=prod /usr/src/app/build ./build
 
 COPY package.json package-lock.json ./
 COPY entrypoint.sh ./
+
+ENTRYPOINT ["/bin/sh"]
+
+CMD ["entrypoint.sh"]