Add checks to verifiy if a new build is needed

This checks if the source materials (python image, Netbox commit,
netbox-docker commit) have changed since the last build. This check is done
by comparing the digest and commit ids from the previous image with the
given tag to the current values taken from the Git and Docker repositories.

The checks are only performed for builds by the automated builds on Github.

Dockerfile

@@ -1,4 +1,4 @@
-ARG FROM=python:3.7-alpine
+ARG FROM
 FROM ${FROM} as builder
 
 RUN apk add --no-cache \

build-functions/get-public-image-config.sh

@@ -0,0 +1,77 @@
+#!/bin/bash
+# Retrieves image configuration from public images in DockerHub
+# Functions from https://gist.github.com/cirocosta/17ea17be7ac11594cb0f290b0a3ac0d1
+# Optimised for our use case
+
+get_image_label() {
+  local label=$1
+  local image=$2
+  local tag=$3
+  local token=$(_get_token $image)
+  local digest=$(_get_digest $image $tag $token)
+  local retval="null"
+  if [ $digest != "null" ]; then
+    retval=$(_get_image_configuration $image $token $digest $label)
+  fi
+  echo $retval
+}
+
+get_image_layers() {
+  local image=$1
+  local tag=$2
+  local token=$(_get_token $image)
+  _get_layers $image $tag $token
+}
+
+get_image_last_layer() {
+  local image=$1
+  local tag=$2
+  local token=$(_get_token $image)
+  local layers=($(_get_layers $image $tag $token))
+  echo ${layers[-1]}
+}
+
+_get_image_configuration() {
+  local image=$1
+  local token=$2
+  local digest=$3
+  local label=$4
+  curl \
+    --silent \
+    --location \
+    --header "Authorization: Bearer $token" \
+    "https://registry-1.docker.io/v2/$image/blobs/$digest" \
+    | jq -r ".config.Labels.\"$label\""
+}
+
+_get_token() {
+  local image=$1
+  curl \
+    --silent \
+    "https://auth.docker.io/token?scope=repository:$image:pull&service=registry.docker.io" \
+    | jq -r '.token'
+}
+
+_get_digest() {
+  local image=$1
+  local tag=$2
+  local token=$3
+  curl \
+    --silent \
+    --header "Accept: application/vnd.docker.distribution.manifest.v2+json" \
+    --header "Authorization: Bearer $token" \
+    "https://registry-1.docker.io/v2/$image/manifests/$tag" \
+    | jq -r '.config.digest'
+}
+
+_get_layers() {
+  local image=$1
+  local tag=$2
+  local token=$3
+  curl \
+    --silent \
+    --header "Accept: application/vnd.docker.distribution.manifest.v2+json" \
+    --header "Authorization: Bearer $token" \
+    "https://registry-1.docker.io/v2/$image/manifests/$tag" \
+    | jq -r '.layers[].digest'
+}

build.sh

@@ -49,7 +49,7 @@ if [ "${1}x" == "x" ] || [ "${1}" == "--help" ] || [ "${1}" == "-h" ]; then
   echo "  DOCKERFILE  The name of Dockerfile to use."
   echo "              Default: Dockerfile"
   echo "  DOCKER_FROM The base image to use."
-  echo "              Default: Whatever is defined as default in the Dockerfile."
+  echo "              Default: 'python:3.7-alpine'"
   echo "  DOCKER_TARGET A specific target to build."
   echo "              It's currently not possible to pass multiple targets."
   echo "              Default: main ldap"
@@ -153,6 +153,11 @@ if [ ! -f "${DOCKERFILE}" ]; then
   fi
 fi
 
+###
+# Determining the value for DOCKER_FROM
+###
+DOCKER_FROM="${DOCKER_FROM-python:3.7-alpine}"
+
 ###
 # Variables for labelling the docker image
 ###
@@ -233,6 +238,49 @@ for DOCKER_TARGET in "${DOCKER_TARGETS[@]}"; do
   # Proceeding to buils stage, except if `--push-only` is passed
   ###
   if [ "${2}" != "--push-only" ] ; then
+    ###
+    # Checking if the build is necessary,
+    # meaning build only if one of those values changed:
+    # - Python base image digest (Label: PYTHON_BASE_DIGEST)
+    # - netbox git ref (Label: NETBOX_GIT_REF)
+    # - netbox-docker git ref (Label: org.label-schema.vcs-ref)
+    ###
+    # Load information from registry (only for docker.io)
+    SHOULD_BUILD="false"
+    BUILD_REASON=""
+    if [ -z "${GH_ACTION}" ]; then
+      # Asuming non Github builds should always proceed
+      SHOULD_BUILD="true"
+      BUILD_REASON="${BUILD_REASON} interactive"
+    fi
+    if [ $DOCKER_REGISTRY = "docker.io" ] && [ $SHOULD_BUILD = "false" ]; then
+      source ./build-functions/get-public-image-config.sh
+      IFS=':' read -ra DOCKER_FROM_SPLIT <<< "${DOCKER_FROM}"
+      if ! [[ ${DOCKER_FROM_SPLIT[0]} =~ ".*/.*" ]]; then
+        # Need to use "library/..." for images the have no two part name
+        DOCKER_FROM_SPLIT[0]="library/${DOCKER_FROM_SPLIT[0]}"
+      fi
+      PYTHON_LAST_LAYER=$(get_image_last_layer ${DOCKER_FROM_SPLIT[0]} ${DOCKER_FROM_SPLIT[1]})
+      IMAGES_LAYERS_OLD=($(get_image_layers ${DOCKER_ORG}/${DOCKER_REPO} ${TAG}))
+      NETBOX_GIT_REF_OLD=$(get_image_label NETBOX_GIT_REF ${DOCKER_ORG}/${DOCKER_REPO} ${TAG})
+      GIT_REF_OLD=$(get_image_label org.label-schema.vcs-ref ${DOCKER_ORG}/${DOCKER_REPO} ${TAG})
+
+      if ! printf '%s\n' ${IMAGES_LAYERS_OLD[@]} | grep -q -P "^${PYTHON_LAST_LAYER}\$"; then
+        SHOULD_BUILD="true"
+        BUILD_REASON="${BUILD_REASON} python"
+      fi
+      if [ "${NETBOX_GIT_REF}" != "${NETBOX_GIT_REF_OLD}" ]; then
+        SHOULD_BUILD="true"
+        BUILD_REASON="${BUILD_REASON} netbox"
+      fi
+      if [ "${GIT_REF}" != "${GIT_REF_OLD}" ]; then
+        SHOULD_BUILD="true"
+        BUILD_REASON="${BUILD_REASON} netbox-docker"
+      fi
+    else
+      SHOULD_BUILD="true"
+      BUILD_REASON="${BUILD_REASON} no-check"
+    fi
     ###
     # Composing all arguments for `docker build`
     ###
@@ -269,6 +317,10 @@ for DOCKER_TARGET in "${DOCKER_TARGETS[@]}"; do
         --label "NETBOX_GIT_URL=${NETBOX_GIT_URL}"
       )
     fi
+    if [ -n "${BUILD_REASON}" ]; then
+      BUILD_REASON=$(sed -e 's/^[[:space:]]*//' -e 's/[[:space:]]*$//' <<< "$BUILD_REASON")
+      DOCKER_BUILD_ARGS+=( --label "BUILD_REASON=${BUILD_REASON}" )
+    fi
 
     # --build-arg
     DOCKER_BUILD_ARGS+=(   --build-arg "NETBOX_PATH=${NETBOX_PATH}" )
@@ -287,9 +339,15 @@ for DOCKER_TARGET in "${DOCKER_TARGETS[@]}"; do
     ###
     # Building the docker image
     ###
+    if [ "${SHOULD_BUILD}" == "true" ]; then
       echo "🐳 Building the Docker image '${TARGET_DOCKER_TAG}'."
+      echo "    Build reason set to: ${BUILD_REASON}"
       $DRY docker build "${DOCKER_BUILD_ARGS[@]}" .
       echo "✅ Finished building the Docker images '${TARGET_DOCKER_TAG}'"
+    else
+      echo "Build skipped because sources didn't change"
+      echo "::set-output name=skipped::true"
+    fi
   fi
 
   ###