linux-fresh: Set DEBIAN_FRONTEND as environment variable

Prevents repeating the same code text over and over each time apt-get is
used. This is a required variable to use apt-get in a non-interactive
container, so it should be set globally, anyways.

linux-fresh/Dockerfile

@@ -3,6 +3,7 @@ MAINTAINER yuzu
 
 ENV CLANG_VER=12
 ENV CMAKE_VER=3.16.3
+ENV DEBIAN_FRONTEND=noninteractive
 ENV GCC_VER=10
 ENV QT_PKG_VER=512
 ENV QT_VER=5.12.6
@@ -10,8 +11,8 @@ ENV UBUNTU_VER=bionic
 
 # Create a user account yuzu (UID 1027) that the container will run as
 RUN useradd -m -u 1027 -s /bin/bash yuzu && \
-    DEBIAN_FRONTEND=noninteractive apt-get update && apt-get -y full-upgrade && \
-    DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
+    apt-get update && apt-get -y full-upgrade && \
+    apt-get install --no-install-recommends -y \
     build-essential \
     ccache \
     file \
@@ -38,7 +39,7 @@ RUN add-apt-repository -y ppa:beineri/opt-qt-${QT_VER}-${UBUNTU_VER} && \
     add-apt-repository -y ppa:jonathonf/ffmpeg-4 && \
     add-apt-repository -y ppa:ubuntu-toolchain-r/test && \
     apt-get update -y && \
-    DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
+    apt-get install --no-install-recommends -y \
     g++-${GCC_VER} \
     gcc-${GCC_VER} \
     libavcodec-dev \
@@ -54,7 +55,7 @@ RUN add-apt-repository -y ppa:beineri/opt-qt-${QT_VER}-${UBUNTU_VER} && \
 RUN wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | apt-key add - && \
     echo "deb http://apt.llvm.org/${UBUNTU_VER}/ llvm-toolchain-${UBUNTU_VER}-${CLANG_VER} main" >> /etc/apt/sources.list && \
     apt-get update -y && \
-    DEBIAN_FRONTEND=noninteractive apt-get install --no-install-recommends -y \
+    apt-get install --no-install-recommends -y \
     clang-${CLANG_VER} && \
     ln -s $(which clang-${CLANG_VER}) /usr/bin/clang && \
     ln -s $(which clang++-${CLANG_VER}) /usr/bin/clang++ && \