Set up a CI job to build and test the Debian packaging

Refs [ticket: 4150](https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4150)

.circleci/config.yml

@@ -117,6 +117,10 @@ workflows:
       - "another-locale":
           {}
 
+      # Test our sources with the packaging for Debian Testing
+      # https://tahoe-lafs.org/trac/tahoe-lafs/ticket/4150
+      - "debian-trixie-package"
+
       - "windows-server-2022":
           name: "Windows Server 2022, CPython <<matrix.pythonVersion>>"
           matrix:
@@ -542,6 +546,60 @@ jobs:
       <<: *UTF_8_ENVIRONMENT
       TAHOE_LAFS_TOX_ENVIRONMENT: "py310"
 
+  debian-trixie-package:
+    docker:
+      - <<: *DOCKERHUB_AUTH
+        image: "debian:trixie"
+    steps:
+      - run:
+          name: "Get latest Debian repository index and install essentials"
+          command: |
+            apt update
+            apt-get install --no-install-recommends --yes \
+              git \
+              ca-certificates \
+              quilt
+      - run:
+          name: "Download sources for Tahoe-LAFS and @merkys Debian packaging"
+          command: |
+            mkdir /tmp/debian-trixie-package
+            cd /tmp/debian-trixie-package
+            # Take Tahoe-LAFS source
+            git clone https://github.com/tahoe-lafs/tahoe-lafs.git
+            # Take debian/ directory from Debian packaging repository
+            git clone https://salsa.debian.org/merkys/tahoe-lafs.git merkys-tahoe-lafs
+            cd tahoe-lafs/
+            ln -s ../merkys-tahoe-lafs/debian
+            ln -s ../merkys-tahoe-lafs/debian/patches
+      - run:
+          name: "Install Tahoe-LAFS dependencies from the Debian repositories"
+          command: |
+            # 'apt-get install' dependencies from debian/control
+            apt-get install --no-install-recommends --yes $(awk '
+              /^(Build-)?Depends:/ || /^ / && deps {
+                sub(/^[^ ]+: /, "")
+                deps = 1
+                dep_str = dep_str ", " $0
+                next
+              }
+              { deps=0 }
+              END {
+                split(dep_str, dep_array, /[,|] */)
+                for (d in dep_array) {
+                  dep = dep_array[d]
+                  gsub(/[^a-z0-9_.+-].*$/, "", dep)
+                  if (dep && !seen[dep]++) print dep
+                }
+              }' debian/control)
+      - run:
+          name: "Apply the Debian package's patches"
+          command: |
+            quilt push -a
+      - run:
+          name: "Build & run tests"
+          command: |
+            make -f debian/rules binary
+
   nixos:
     parameters:
       nixpkgs: