drone: Switch to Debian Buster-based Docker image

Many users are having trouble using the CI-built toolchains because they
are using the latest Ubuntu LTS release (18.04 at the time of writing)
which has glibc 2.27, while the Fedora 31 build image has glibc 2.29:

clang: /lib/x86_64-linux-gnu/libm.so.6: version `GLIBC_2.29' not found (required by clang)
clang: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.26' not found (required by clang)

Build with an image based on Debian Buster instead, which has glibc 2.28
and is compatible with Ubuntu's ancient glibc 2.27 while maintaining
newer versions of other packages. This also allows us to shrink the
build image thanks to Bitnami's excellent minimal bitnami/minideb Debian
base that removes unnecessary files from the container after package
installation.

Our Debian Buster image is currently 652 MB, while the Fedora 31 image
is 1.07 GB -- a significant reduction in size that should help reduce
build times a bit.

Signed-off-by: Danny Lin <[email protected]>

.drone/drone.yml

@@ -23,7 +23,7 @@ steps:
           - tag
 
   - name: build
-    image: kdrag0n/fedora-llvm-build:clang-8.0.0
+    image: kdrag0n/debian-llvm-build:buster
 
     commands:
       - .drone/build.sh
@@ -34,7 +34,7 @@ steps:
           - tag
 
   - name: github
-    image: kdrag0n/fedora-llvm-build:clang-8.0.0
+    image: kdrag0n/debian-llvm-build:buster
 
     environment:
       # GitHub personal access token ()
@@ -56,7 +56,7 @@ steps:
           - tag
 
   - name: telegram
-    image: kdrag0n/fedora-llvm-build:clang-8.0.0
+    image: kdrag0n/debian-llvm-build:buster
 
     environment:
       # Telegram chat ID
@@ -83,7 +83,7 @@ steps:
           - tag
 
   - name: debug-wait
-    image: kdrag0n/fedora-llvm-build:clang-8.0.0
+    image: kdrag0n/debian-llvm-build:buster
 
     environment:
       # Amount of time to wait for debugging