fix docker build for the armebv7r-none-eabi target

Author: japaric

src/ci/docker/dist-various-1/Dockerfile

@@ -111,7 +111,8 @@ ENV TARGETS=$TARGETS,armv7r-none-eabihf
 ENV CC_mipsel_unknown_linux_musl=mipsel-openwrt-linux-gcc \
     CC_mips_unknown_linux_musl=mips-openwrt-linux-gcc \
     CC_sparc64_unknown_linux_gnu=sparc64-linux-gnu-gcc \
-    CC_x86_64_unknown_redox=x86_64-unknown-redox-gcc
+    CC_x86_64_unknown_redox=x86_64-unknown-redox-gcc \
+    CC_armebv7r_none_eabi=arm-none-eabi-gcc
 
 ENV RUST_CONFIGURE_ARGS \
       --musl-root-armv5te=/musl-armv5te \