Add mount cache for docker building

[lunny]

This PR use cache mounts feature of docker to accelerate the docker image build process.
ref: https://docs.docker.com/build/cache/optimize/#use-cache-mounts

Run docker build locally it will take about 242.8s and now it's 16s after cached.

TODO:

• Add shared cache in CI/CD

    cache-from: type=registry,ref=gitea/gitea:buildcache-v1.24
    cache-to: type=registry,ref=gitea/gitea:buildcache-v1.24,mode=max

[wxiaoguang]

-> Reduce incremental Docker build times #27998

[wxiaoguang]

One more thing, the rootful and rootless can share the some build and binaries. For example, build rootless first (or a common image first), then copy its binaries (gitea, environment-to-ini) to the rootful image.

(Just a hint, not a must)