diff --git a/Dockerfile b/Dockerfile
index e327641..212517c 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -48,10 +48,10 @@ USER ${AVATAO_USER}
 
 WORKDIR /home/${AVATAO_USER}
 COPY .pyenvrc .
-ENV PYTHON_VERSION="3.6.4"
+ARG PYTHON_VERSION="3.6.4"
 # no GitHub releases are available, so current tip of head is hardcoded to avoid surprises
-ENV PYENV_INSTALLER_URL="https://raw.githubusercontent.com/pyenv/pyenv-installer/78cfd4d/bin/pyenv-installer"
-ENV PYENV_INSTALLER_HASH=9509348b828f0564358fff456f7f693dd9ace351dc3f240854d7685ad8a8e1dd
+ARG PYENV_INSTALLER_URL="https://raw.githubusercontent.com/pyenv/pyenv-installer/78cfd4d/bin/pyenv-installer"
+ARG PYENV_INSTALLER_HASH=9509348b828f0564358fff456f7f693dd9ace351dc3f240854d7685ad8a8e1dd
 RUN curl -fSL -o pyenv-installer ${PYENV_INSTALLER_URL} && \
     echo "${PYENV_INSTALLER_HASH} *pyenv-installer" | sha256sum -c - && \
     bash pyenv-installer && \