Docker base image: fix basic cross compile

[Frenzie]

Cf. koreader/koreader-base#1375 (comment)

Perhaps I overlooked something in what I tried previously but I'm not inclined to try to figure out what might be wrong if it Just Works™ with the default GCC 7 TCs.

diff --git a/docker/ubuntu/baseimage/bootstrap.sh b/docker/ubuntu/baseimage/bootstrap.sh
index 07a782d..9d8f669 100755
--- a/docker/ubuntu/baseimage/bootstrap.sh
+++ b/docker/ubuntu/baseimage/bootstrap.sh
@@ -40,8 +40,12 @@ apt-get install --no-install-recommends -y \
     "${ARM_SF_TC[@]}" "${ARM_HF_TC[@]}" "${ARM64_TC[@]}" \
     "${LUAJIT_DEPS[@]}"
 
-sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 700 --slave /usr/bin/g++ g++ /usr/bin/g++-7
-sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 800 --slave /usr/bin/g++ g++ /usr/bin/g++-8
+update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 700 --slave /usr/bin/g++ g++ /usr/bin/g++-7
+update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 800 --slave /usr/bin/g++ g++ /usr/bin/g++-8
+
+update-alternatives --install /usr/bin/arm-linux-gnueabi-gcc arm-linux-gnueabi-gcc /usr/bin/arm-linux-gnueabi-gcc-8 100 --slave /usr/bin/arm-linux-gnueabi-g++ arm-linux-gnueabi-g++ /usr/bin/arm-linux-gnueabi-g++-8
+update-alternatives --install /usr/bin/arm-linux-gnueabihf-gcc arm-linux-gnueabihf-gcc /usr/bin/arm-linux-gnueabihf-gcc-8 100 --slave /usr/bin/arm-linux-gnueabihf-g++ arm-linux-gnueabihf-g++ /usr/bin/arm-linux-gnueabihf-g++-8
+update-alternatives --install /usr/bin/aarch64-linux-gnu-gcc aarch64-linux-gnu-gcc /usr/bin/aarch64-linux-gnu-gcc-8 100 --slave /usr/bin/aarch64-linux-gnu-g++ aarch64-linux-gnu-g++ /usr/bin/aarch64-linux-gnu-g++-8

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This change is