enable keypair auth and some refactoring

.travis.yml

@@ -0,0 +1,36 @@
+sudo: required
+
+language: bash
+
+addons:
+  apt:
+    packages:
+      - docker-ce
+      - w3m
+
+env:
+  global:
+    - IMAGE="alpine-sshd"
+    - IMAGE_NAME="${DOCKER_USER}/${IMAGE}"
+
+before_script:
+  - sudo sysctl -w net.ipv4.ip_forward=1
+  - docker login -u "${DOCKER_USER}" -p "${DOCKER_PASSWORD}"
+  - export ALPINE_VERSION="3.8"
+  - export OPENSSH_VERSION="$(w3m -dump "https://pkgs.alpinelinux.org/packages?name=openssh&branch=v${ALPINE_VERSION}" | grep -m 1 "x86" | awk '{print $2}')"
+
+script: |
+  docker build \
+          --no-cache \
+          --pull \
+          --quiet \
+          --build-arg ALPINE_VERSION="${ALPINE_VERSION}" \
+          --build-arg OPENSSH_VERSION="${OPENSSH_VERSION}" \
+          --tag "${IMAGE_NAME}:${OPENSSH_VERSION}-alpine{ALPINE_VERSION}" \
+          --tag "${IMAGE_NAME}:latest" \
+          --file "${TRAVIS_BUILD_DIR}/Dockerfile" \
+          "${TRAVIS_BUILD_DIR}"
+  if [[ "${TRAVIS_BRANCH}" == "master" ]] && [[ "${TRAVIS_PULL_REQUEST}" == "false" ]]; then
+    docker push "${IMAGE_NAME}:${OPENSSH_VERSION}-alpine{ALPINE_VERSION}"
+    docker push "${IMAGE_NAME}:latest"
+  fi