Add master branch support for next release to c/build.sh

I believe developing on master would improve the developer experience,
and it can reduce propagation of changes on each release (CI etc?).

For smoother transition master is currently building 8.x branch, we
might want to switch to unreleased 9.x soon.

At the moment master uses Dockerfile-8.x (not duplicated), same with
repo file. Later we could introduce master repo baseurl, and use it in
an unstable Dockerfile.

For the record this change was tested using the following commands:

```
cd ~/mnt/xcp-ng-build-env # https://github.com/xcp-ng/xcp-ng-build-env
./container/build.sh master
cd ~/mnt/xcp-ng-release # https://github.com/xcp-ng-rpms/xcp-ng-release
~/mnt/xcp-ng-build-env/src/xcp_ng_dev/cli.py container build master .
```

Signed-off-by: Philippe Coval <philippe.coval@vates.tech>

Author: rzr

container/build.sh

@@ -69,7 +69,16 @@ cd $(dirname "$0")
 
 ALMA_VERSION=
 CENTOS_VERSION=
-case "$1" in
+XCP_NG_BRANCH="$1"
+IMAGE_BRANCH="${XCP_NG_BRANCH}"
+
+case "$XCP_NG_BRANCH" in
+    master)
+        # TODO: keep aligned to next release
+        XCP_NG_BRANCH=8.3
+        DOCKERFILE=Dockerfile-8.x
+        : ${PLATFORM:=linux/amd64}
+        ;;
     9.*)
         DOCKERFILE=Dockerfile-9.x
         ALMA_VERSION=10.0
@@ -80,14 +89,14 @@ case "$1" in
         : ${PLATFORM:=linux/amd64}
         ;;
     *)
-        echo >&2 "Unsupported release '$1'"
+        echo >&2 "Unsupported release '${XCP_NG_BRANCH}'"
         exit 1
         ;;
 esac
 
 "$RUNNER" build \
     --platform "$PLATFORM" \
-    -t ghcr.io/xcp-ng/xcp-ng-build-env:${1} \
-    --build-arg XCP_NG_BRANCH=${1} \
+    -t ghcr.io/xcp-ng/xcp-ng-build-env:${IMAGE_BRANCH} \
+    --build-arg XCP_NG_BRANCH=${XCP_NG_BRANCH} \
     --ulimit nofile=1024 \
     -f $DOCKERFILE .