Move environment variable where it belongs.

Dockerfile

@@ -6,6 +6,7 @@ ARG DEBIAN_FRONTEND=noninteractive
 ENV PYTHONUNBUFFERED 1
 ENV PYTHONPATH $PYTHONPATH:/webodm
 ENV PROJ_LIB=/usr/share/proj
+ENV NODE_MAJOR=20
 
 # Copy WebODM files
 ADD . /webodm/
@@ -35,7 +36,6 @@ RUN apt-get -qq update && \
 # Node.js deb source
 RUN mkdir -p /etc/apt/keyrings && \
     curl -fsSL https://deb.nodesource.com/gpgkey/nodesource-repo.gpg.key | gpg --dearmor -o /etc/apt/keyrings/nodesource.gpg && \
-    NODE_MAJOR=20 && \
     echo "deb [signed-by=/etc/apt/keyrings/nodesource.gpg] https://deb.nodesource.com/node_$NODE_MAJOR.x nodistro main" | tee /etc/apt/sources.list.d/nodesource.list && \
     apt-get -qq update && \
     apt-get -qq install -y nodejs