Migrate neco-admission to ghcr.io

[yokaze]

This PR:

• adds build_admission action to build neco-admission
• adds extract_tags action to extract the next BRANCH and TAG and check their consistency
  • it exposes 5 outputs to use:
    • branch: image branch
    • tag: image tag
    • build: if we need to build the container
    • push: if we need to upload the container
    • docker_push: same as push, but used as the input of docker/build-push-action
• makes build_admission and build_push use extract_tags
• updates bmc-reverse-proxy to show a sample CI result

extract_tags echos its decision in CI output: for example,

• nop

vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv
    Build Policy:

         branch: 
            tag: 
          build: 
           push: 
    docker_push: false
^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^

• build only

vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv
    Build Policy:

         branch: 
            tag: ghcr.io/cybozu/bmc-reverse-proxy:0.1.25
          build: true
           push: 
    docker_push: false
^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^

• build and push

vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv vvvvvvvvvv
    Build Policy:

         branch: 
            tag: ghcr.io/cybozu/admission:0.24.2
          build: true
           push: true
    docker_push: true
^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^

(Yes I've mistakenly uploaded ghcr.io/cybozu/admission:0.24.2 during the work for this PR...)
https://github.com/cybozu/neco-containers/actions/runs/7283597673/job/19847696899

Signed-off-by: Daichi Sakaue [email protected]

[zeroalphat]

LGTM