Minor fix to gitpod dockerfile [skip ci] (ddev#3969)

Author: rfay

.gitpod/images/Dockerfile

@@ -33,4 +33,4 @@ RUN for item in golang.org/x/tools/gopls@latest github.com/go-delve/delve/cmd/dl
     done
 RUN cp ~/go/bin/dlv ~/go/bin/dlv-dap
 
-RUN pip3 install -r docs/mkdocs-pip-requirements
+RUN cd /tmp && curl -LO --fail https://raw.githubusercontent.com/drud/ddev/master/docs/mkdocs-pip-requirements && pip3 install -r /tmp/mkdocs-pip-requirements