github: convert all Linux jobs to be containerized

We have split the CI jobs in GitHub Workflows into two categories:

  - Those running on a machine pool directly.

  - Those running in a container on the machine pool.

The latter is more flexible because it allows us to freely pick whatever
container image we want to use for a specific job, while the former only
allows us to pick from a handful of different distros. The containerized
jobs do not have any significant downsides to the best of my knowledge:

  - They aren't significantly slower to start up. A quick comparison by
    Peff shows that the difference is mostly lost in the noise:

            job             |  old | new
        --------------------|------|------
        linux-TEST-vars      11m30s 10m54s
        linux-asan-ubsan     30m26s 31m14s
        linux-gcc             9m47s 10m6s
        linux-gcc-default     9m47s  9m41s
        linux-leaks          25m50s 25m21s
        linux-meson          10m36s 10m41s
        linux-reftable       10m25s 10m23s
        linux-reftable-leaks 27m18s 27m28s
        linux-sha256          9m54s 10m31s

    Some jobs are a bit faster, some are a bit slower, but there does
    not seem to be any significant change.

  - Containerized jobs run as root, which keeps a couple of tests from
    running. This has been addressed in the preceding commit though,
    where we now use setpriv(1) to run tests as a separate user.

  - GitHub injects a Node binary into containerized jobs, which is
    dynamically linked. This has led to some issues in the past [1], but
    only for our 32 bit jobs. The issues have since been resolved.

Overall there seem to be no downsides, but the upside is that we have
more control over the exact image that these jobs use. Convert the Linux
jobs accordingly.

[1]: https://lore.kernel.org/git/20240912094841.GD589828@coredump.intra.peff.net/

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

.github/workflows/main.yml

@@ -259,20 +259,6 @@ jobs:
       fail-fast: false
       matrix:
         vector:
-          - jobname: linux-sha256
-            cc: clang
-            pool: ubuntu-latest
-          - jobname: linux-reftable
-            cc: clang
-            pool: ubuntu-latest
-          - jobname: linux-gcc
-            cc: gcc
-            cc_package: gcc-8
-            pool: ubuntu-20.04
-          - jobname: linux-TEST-vars
-            cc: gcc
-            cc_package: gcc-8
-            pool: ubuntu-20.04
           - jobname: osx-clang
             cc: clang
             pool: macos-13
@@ -285,21 +271,6 @@ jobs:
           - jobname: osx-meson
             cc: clang
             pool: macos-13
-          - jobname: linux-gcc-default
-            cc: gcc
-            pool: ubuntu-latest
-          - jobname: linux-leaks
-            cc: gcc
-            pool: ubuntu-latest
-          - jobname: linux-reftable-leaks
-            cc: gcc
-            pool: ubuntu-latest
-          - jobname: linux-asan-ubsan
-            cc: clang
-            pool: ubuntu-latest
-          - jobname: linux-meson
-            cc: gcc
-            pool: ubuntu-latest
     env:
       CC: ${{matrix.vector.cc}}
       CC_PACKAGE: ${{matrix.vector.cc_package}}
@@ -342,6 +313,44 @@ jobs:
       fail-fast: false
       matrix:
         vector:
+        - jobname: linux-sha256
+          image: ubuntu:latest
+          cc: clang
+          distro: ubuntu-latest
+        - jobname: linux-reftable
+          image: ubuntu:latest
+          cc: clang
+          distro: ubuntu-latest
+        - jobname: linux-gcc
+          image: ubuntu:20.04
+          cc: gcc
+          cc_package: gcc-8
+          distro: ubuntu-20.04
+        - jobname: linux-TEST-vars
+          image: ubuntu:20.04
+          cc: gcc
+          cc_package: gcc-8
+          distro: ubuntu-20.04
+        - jobname: linux-gcc-default
+          image: ubuntu:latest
+          cc: gcc
+          distro: ubuntu-latest
+        - jobname: linux-leaks
+          image: ubuntu:latest
+          cc: gcc
+          distro: ubuntu-latest
+        - jobname: linux-reftable-leaks
+          image: ubuntu:latest
+          cc: gcc
+          distro: ubuntu-latest
+        - jobname: linux-asan-ubsan
+          image: ubuntu:latest
+          cc: clang
+          distro: ubuntu-latest
+        - jobname: linux-meson
+          image: ubuntu:latest
+          cc: gcc
+          distro: ubuntu-latest
         - jobname: linux-musl
           image: alpine
           distro: alpine-latest
@@ -363,6 +372,7 @@ jobs:
     env:
       jobname: ${{matrix.vector.jobname}}
       distro: ${{matrix.vector.distro}}
+      CC: ${{matrix.vector.cc}}
     runs-on: ubuntu-latest
     container: ${{matrix.vector.image}}
     steps: