packaging: make dockerrpm fedora target more generic

Fedora moves fast in version numbers, and often with Mercurial packaging being
backwards compatible. Thus, only aim for providing built-in support for latest
Fedora version, and make it easy to update.

With this refactoring, 'dockerrpm fedora31' also works.

'dockerrpm fedora' will use the 'fedora:latest' Docker image.

contrib/packaging/docker/fedora.template

@@ -1,4 +1,4 b''
-FROM fedora:29
+FROM fedora:%OS_RELEASE%
 
 RUN groupadd -g 1000 build && \
     useradd -u 1000 -g 1000 -s /bin/bash -d /build -m build

contrib/packaging/dockerrpm

@@ -6,6 +6,14 b' export ROOTDIR=$(cd $BUILDDIR/../..; pwd'
 PLATFORM="$1"
 shift # extra params are passed to buildrpm
 
+DOCKERFILE="$PLATFORM"
+OS_RELEASE="${PLATFORM//[a-z]/}"
+case "$PLATFORM" in
+fedora*)
+    DOCKERFILE="${PLATFORM//[0-9]/}.template"
+    ;;
+esac
+
 DOCKER=$($BUILDDIR/hg-docker docker-path)
 
 CONTAINER=hg-docker-$PLATFORM
@@ -18,7 +26,11 b' else'
     DOCKERGID=$(id -g)
 fi
 
-$BUILDDIR/hg-docker build --build-arg UID=$DOCKERUID --build-arg GID=$DOCKERGID $BUILDDIR/docker/$PLATFORM $CONTAINER
+$BUILDDIR/hg-docker build \
+    --build-arg UID=$DOCKERUID \
+    --build-arg GID=$DOCKERGID \
+    --build-arg OS_RELEASE=${OS_RELEASE:-latest} \
+    $BUILDDIR/docker/$DOCKERFILE $CONTAINER
 
 RPMBUILDDIR=$ROOTDIR/packages/$PLATFORM
 mkdir -p $RPMBUILDDIR