diff --git a/.cirrus.yml b/.cirrus.yml
index 3b7e5f2823f6d..ac33507733897 100644
--- a/.cirrus.yml
+++ b/.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:
diff --git a/ci/cirrus.sh b/ci/cirrus.sh
index e10037a468474..afd72b493c999 100755
--- a/ci/cirrus.sh
+++ b/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" \
diff --git a/ci/linux-debian-s390-qemu.Dockerfile b/ci/linux-debian-s390-qemu.Dockerfile
deleted file mode 100644
index d527be55cb23f..0000000000000
--- a/ci/linux-debian-s390-qemu.Dockerfile
+++ /dev/null
@@ -1,6 +0,0 @@
-FROM debian
-
-RUN dpkg --add-architecture s390x
-RUN apt-get update
-RUN apt-get install --no-install-recommends --no-upgrade -y make automake libtool
-RUN apt-get install --no-install-recommends --no-upgrade -y gcc-s390x-linux-gnu libc6-dev-s390x-cross qemu-user libc6:s390x
diff --git a/ci/linux-debian.Dockerfile b/ci/linux-debian.Dockerfile
new file mode 100644
index 0000000000000..8fe50f17e0651
--- /dev/null
+++ b/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
diff --git a/ci/linux-nixos.Dockerfile b/ci/linux-nixos.Dockerfile
deleted file mode 100644
index 0017073c9dc3d..0000000000000
--- a/ci/linux-nixos.Dockerfile
+++ /dev/null
@@ -1,12 +0,0 @@
-FROM nixos/nix
-
-COPY ci/shell.nix /tmp
-COPY ci/shell-i686.nix /tmp
-
-RUN nix-channel --remove nixpkgs
-RUN nix-channel --add https://nixos.org/channels/nixos-20.09 nixpkgs
-RUN nix-channel --update
-
-# Run dummy command "true" in the nix-shell just to get the packages prepared.
-RUN nix-shell /tmp/shell.nix --command true
-RUN nix-shell /tmp/shell-i686.nix --command true
diff --git a/ci/mkshell.nix b/ci/mkshell.nix
deleted file mode 100644
index 3886b556d0997..0000000000000
--- a/ci/mkshell.nix
+++ /dev/null
@@ -1,12 +0,0 @@
-{ pkgs }:
-
-with pkgs;
-
-mkShell {
-   buildInputs = [
-       bash file pkgconfig autoconf automake libtool gmp valgrind clang gcc
-   ];
-   shellHook = ''
-       echo Running nix-shell with nixpkgs version: $(nix eval --raw nixpkgs.lib.version)
-   '';
-}
diff --git a/ci/shell-i686.nix b/ci/shell-i686.nix
deleted file mode 100644
index 12528dd9f005f..0000000000000
--- a/ci/shell-i686.nix
+++ /dev/null
@@ -1,4 +0,0 @@
-let
-  pkgs = (import <nixpkgs> {}).pkgsi686Linux;
-in
-import ./mkshell.nix { inherit pkgs; }
diff --git a/ci/shell.nix b/ci/shell.nix
deleted file mode 100644
index e83dc870f0442..0000000000000
--- a/ci/shell.nix
+++ /dev/null
@@ -1,4 +0,0 @@
-let
-  pkgs = (import <nixpkgs> {});
-in
-import ./mkshell.nix { inherit pkgs; }