Switch to Docker Hub and use shared runners

- Change registry from GitLab to Docker Hub (gmcintire/towerops-agent)
- Update authentication to use DOCKERHUB_USERNAME and DOCKERHUB_TOKEN
- Remove vntx runner tags to use GitLab shared runners

.gitlab-ci.yml

@@ -17,8 +17,6 @@ variables:
 # Test stage - compile and check code
 test:
   stage: test
-  tags:
-    - vntx
   image: rust:1.83-alpine
   before_script:
     - apk add --no-cache musl-dev
@@ -39,8 +37,6 @@ test:
 # Build Docker image for branches (test build)
 build:test:
   stage: build
-  tags:
-    - vntx
   image: docker:24-dind
   services:
     - docker:24-dind
@@ -63,8 +59,6 @@ build:test:
 # Build and push for main branch
 build:main:
   stage: build
-  tags:
-    - vntx
   image: docker:24-dind
   services:
     - docker:24-dind
@@ -88,8 +82,6 @@ build:main:
 # Release build for tags (versioned releases)
 release:
   stage: release
-  tags:
-    - vntx
   image: docker:24-dind
   services:
     - docker:24-dind
@@ -122,8 +114,6 @@ release:
 
 build:multiarch:
   stage: build
-  tags:
-    - vntx
   image: docker:24-dind
   services:
     - docker:24-dind