[RELEASE] decouple the release name from the version number

The release name, as provided by FORGEJO_RELEASE, is used to build OCI
images and binary files. Although it can be the same as the Forgejo
version, it is not a requirement.

When the FORGEJO_RELEASE environment variable is set, use it as a
default for naming the binary file instead of FORGEJO_VERSION. For
instance, when building from the forgejo branch here is what is desired:

FORGEJO_VERSION=7.0.0-g2343
GITEA_VERSION=1.22.0
VERSION=vforgejo-test

The name of the release is also displayed with forgejo --version
for sanity check purposes.

Before:

FORGEJO_VERSION is the computed version
GITEA_VERSION is set manually
VERSION defaults to FORGEJO_VERSION
forgejo --help does not display VERSION

After:

FORGEJO_VERSION is the computed version
GITEA_VERSION is set manually
RELEASE_VERSION defaults to FORGEJO_VERSION
VERSION defaults to RELEASE_VERSION
forgejo --help displays VERSION

Dockerfile.rootless

@@ -5,7 +5,7 @@ FROM --platform=$BUILDPLATFORM docker.io/library/golang:1.21-alpine3.19 as build
 ARG GOPROXY
 ENV GOPROXY ${GOPROXY:-direct}
 
-ARG GITEA_VERSION
+ARG RELEASE_VERSION
 ARG TAGS="sqlite sqlite_unlock_notify"
 ENV TAGS "bindata timetzdata $TAGS"
 ARG CGO_EXTRA_CFLAGS
@@ -36,7 +36,7 @@ WORKDIR ${GOPATH}/src/code.gitea.io/gitea
 RUN make clean-all
 RUN make frontend
 RUN go build contrib/environment-to-ini/environment-to-ini.go && xx-verify environment-to-ini
-RUN make go-check generate-backend static-executable && xx-verify gitea
+RUN make RELEASE_VERSION=$RELEASE_VERSION go-check generate-backend static-executable && xx-verify gitea
 
 # Copy local files
 COPY docker/rootless /tmp/local