Unify nightly naming across binaries and docker images (#24116)

Proposal found here: https://github.com/go-gitea/gitea/issues/23654

TODO: make non-breaking (can we publish docker image using dev and
nightly prefix? at same time). if anyone has advice please comment :)

If this PR is merged, then I can add redirects to the downloads site.

Makefile

 	endif
 endif
 
+# if version = "main" then update version to "nightly"
+ifeq ($(VERSION),main)
+	VERSION := main-nightly
+endif
+
 LDFLAGS := $(LDFLAGS) -X "main.MakeVersion=$(MAKE_VERSION)" -X "main.Version=$(GITEA_VERSION)" -X "main.Tags=$(TAGS)"
 
 LINUX_ARCHS ?= linux/amd64,linux/386,linux/arm-5,linux/arm-6,linux/arm64

docker/manifest.rootless.tmpl

-image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}-rootless
+image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}-rootless
 {{#if build.tags}}
 {{#unless (contains "-rc" build.tag)}}
 tags:
 {{/if}}
 manifests:
   -
-    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}-linux-amd64-rootless
+    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}-linux-amd64-rootless
     platform:
       architecture: amd64
       os: linux
   -
-    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}-linux-arm64-rootless
+    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}-linux-arm64-rootless
     platform:
       architecture: arm64
       os: linux

docker/manifest.tmpl

-image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}
+image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}
 {{#if build.tags}}
 {{#unless (contains "-rc" build.tag)}}
 tags:
 {{/if}}
 manifests:
   -
-    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}-linux-amd64
+    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}-linux-amd64
     platform:
       architecture: amd64
       os: linux
   -
-    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}dev{{/if}}-linux-arm64
+    image: gitea/gitea:{{#if build.tag}}{{trimPrefix "v" build.tag}}{{else}}{{#if (hasPrefix "refs/heads/release/v" build.ref)}}{{trimPrefix "refs/heads/release/v" build.ref}}-{{/if}}nightly{{/if}}-linux-arm64
     platform:
       architecture: arm64
       os: linux