Support Fedora 41 in Dockerfile

This is needed for "make check" to work on a Fedora 41 host where the
FROM directive is overridden to fedora:41.

- Drop the workaround for gdb-headless on F41, it's no longer available
  there and the bug is fixed on F40+:
  https://bugzilla.redhat.com/show_bug.cgi?id=2310156

- Don't try to remove rpm-sign-libs and python3-rpm on F41 since dnf5 no
  longer pulls them in.

- Make sure to call "dnf clean all" before "rpm -e" as python3-rpm is
  needed by dnf4 to function (hence the duplicated call).

- Add a comment at the top saying which Fedora versions are supported by
  the Dockerfile.  This could be checked automatically during cmake
  config but let's leave that for later.

- Detect F41 vs. older by looking for the dnf5 binary, it's the easiest
  way.

tests/Dockerfile.fedora

@@ -1,3 +1,4 @@
+# Supported Fedora releases: 39 40 41
 FROM registry.fedoraproject.org/fedora:40 AS base
 MAINTAINER rpm-maint@lists.rpm.org
 
@@ -46,14 +47,13 @@ RUN dnf -y install \
   tar unzip gzip bzip2 cpio xz p7zip \
   pkgconfig \
   /usr/bin/systemd-sysusers \
-  "gdb-headless < 15" \
   dwz \
   fsverity-utils fsverity-utils-devel \
   pandoc \
   sequoia-sq \
   libasan \
-  libubsan \
-  && dnf clean all
+  libubsan
+
 RUN echo "%_dbpath $(rpm --eval '%_dbpath')" > /root/.rpmmacros
 
 # Workaround for pkgconf(1)'s unlisted dependency on rpm.
@@ -61,12 +61,20 @@ RUN echo "%_dbpath $(rpm --eval '%_dbpath')" > /root/.rpmmacros
 RUN ln -sf $(rpm --eval '%{_target_platform}%{?_gnu}')-pkg-config \
            /usr/bin/pkg-config
 
+# F40 and older (no dnf5) require special handling
+RUN if which dnf5; then \
+        dnf -y install gdb-headless; \
+        dnf clean all; \
+    else \
+        dnf -y install "gdb-headless < 15"; \
+        dnf clean all; \
+        rpm -e --nodeps --nodb rpm-sign-libs python3-rpm; \
+    fi
+
 RUN rpm -e --nodeps --nodb \
 	rpm \
 	rpm-libs \
-	rpm-build-libs \
-	rpm-sign-libs \
-	python3-rpm
+	rpm-build-libs
 
 WORKDIR /
 CMD /bin/bash