ci: Switch all Linux builds to Debian

The experiment of using Nix Shell was not really successful. Most
notably, Nix uses a bunch of wrapper scripts around compilers, which
make the build much less "pure". This may be useful but it's exactly
not what we want for CI. In particular, this resulted in gcc being used
for the "clang" builds because a wrapper script redefined the CC env
variable.

This now builds a single docker image (Debian) for all architectures
that we test in CI on Linux.

.cirrus.yml

@@ -33,9 +33,9 @@ cat_logs_snippet: &CAT_LOGS
       - env
 
 task:
-  name: "x86_64: Linux (Alpine Linux, Nix Shell)"
+  name: "x86_64: Linux (Debian stable)"
   container:
-    dockerfile: ci/linux-nixos.Dockerfile
+    dockerfile: ci/linux-debian.Dockerfile
     # Reduce number of CPUs to be able to do more builds in parallel.
     cpu: 1
     # More than enough for our scripts.
@@ -82,13 +82,13 @@ task:
     - env:
         CC: clang
   test_script:
-    - nix-shell ci/shell.nix --run ./ci/cirrus.sh
+    - ./ci/cirrus.sh
   << : *CAT_LOGS
 
 task:
-  name: "i686: Linux (Alpine Linux, Nix Shell)"
+  name: "i686: Linux (Debian stable)"
   container:
-    dockerfile: ci/linux-nixos.Dockerfile
+    dockerfile: ci/linux-debian.Dockerfile
     cpu: 1
     memory: 1G
   env:
@@ -99,16 +99,16 @@ task:
     SCHNORRSIG: yes
   matrix:
     - env:
-        CC: gcc
+        CC: i686-linux-gnu-gcc
     - env:
-        CC: clang
+        CC: clang --target=i686-pc-linux-gnu -isystem /usr/i686-linux-gnu/include
   matrix:
     - env:
         BIGNUM: gmp
     - env:
         BIGNUM: no
   test_script:
-    - nix-shell ci/shell-i686.nix --run ./ci/cirrus.sh
+    - ./ci/cirrus.sh
   << : *CAT_LOGS
 
 task:
@@ -168,9 +168,9 @@ task:
   << : *CAT_LOGS
 
 task:
-  name: "s390x (big-endian): Linux (Debian QEMU)"
+  name: "s390x (big-endian): Linux (Debian stable, QEMU)"
   container:
-    dockerfile: ci/linux-debian-s390-qemu.Dockerfile
+    dockerfile: ci/linux-debian.Dockerfile
     cpu: 1
     memory: 1G
   env:

ci/cirrus.sh

@@ -12,10 +12,6 @@ valgrind --version || true
 
 ./autogen.sh
 
-# Nix doesn't store GNU file in /usr/bin, see https://lists.gnu.org/archive/html/bug-libtool/2015-09/msg00000.html .
-# The -i'' is necessary for macOS portability, see https://stackoverflow.com/a/4247319 .
-sed -i'' -e 's@/usr/bin/file@$(which file)@g' configure
-
 ./configure \
     --enable-experimental="$EXPERIMENTAL" \
     --with-test-override-wide-multiply="$WIDEMUL" --with-bignum="$BIGNUM" --with-asm="$ASM" \

ci/linux-debian.Dockerfile

@@ -0,0 +1,12 @@
+FROM debian:stable
+
+RUN dpkg --add-architecture i386
+RUN dpkg --add-architecture s390x
+RUN apt-get update
+
+# dkpg-dev: to make pkg-config work in cross-builds
+RUN apt-get install --no-install-recommends --no-upgrade -y \
+        make automake libtool pkg-config dpkg-dev valgrind qemu-user \
+        gcc clang libc6-dbg libgmp-dev \
+        gcc-i686-linux-gnu libc6-dev-i386-cross libc6-dbg:i386 libgmp-dev:i386 \
+        gcc-s390x-linux-gnu libc6-dev-s390x-cross libc6-dbg:s390x