Refactor to multistage builds

This commit introduces a huge change in the build process.

What changed:

- Dockerfile.ldap was integrated into Dockerfile as a seperate
  [build stage][multistage-build].
- All the build scripts were refactored according to this.
- The `docker-compose.yml` file was adjusted likewise.
- The main build script, `/build.sh`, now always builds all
  targets (formerly called variants).
- The minimal requirements for Docker and docker-compose
  have increased.
- The build on hub.docker.com must be adjusted.

This change should also fix #156 permanently.

[multistage-build]: https://docs.docker.com/develop/develop-images/multistage-build/

build-all.sh

@@ -12,8 +12,6 @@ BUILDS=("${BUILD:-"${ALL_BUILDS[@]}"}")
 
 echo "⚙️ Configured builds: ${BUILDS[*]}"
 
-VARIANTS=("main" "ldap")
-
 if [ -n "${DEBUG}" ]; then
   export DEBUG
 fi
@@ -22,63 +20,35 @@ ERROR=0
 
 # Don't build if not on `master` and don't build if on a pull request,
 # but build when DEBUG is not empty
-for VARIANT in "${VARIANTS[@]}"; do
-  export VARIANT
 
-  # Checking which VARIANT to build
-  if [ "${VARIANT}" == "main" ]; then
-    DOCKERFILE="${DOCKERFILE_PATH-Dockerfile}"
-  else
-    DOCKERFILE="${DOCKERFILE_PATH-Dockerfile}.${VARIANT}"
-
-    # Fail fast
-    if [ ! -f "${DOCKERFILE}" ]; then
-      echo "🚨 The Dockerfile '${DOCKERFILE}' for variant '${VARIANT}' doesn't exist."
-      ERROR=1
+for BUILD in "${BUILDS[@]}"; do
+  echo "🛠 Building '$BUILD' from '$DOCKERFILE'"
+  case $BUILD in
+    release)
+      # build the latest release
+      # shellcheck disable=SC2068
+      ./build-latest.sh $@ || ERROR=1
+      ;;
+    prerelease)
+      # build the latest pre-release
+      # shellcheck disable=SC2068
+      PRERELEASE=true ./build-latest.sh $@ || ERROR=1
+      ;;
+    branches)
+      # build all branches
+      # shellcheck disable=SC2068
+      ./build-branches.sh $@ || ERROR=1
+      ;;
+    *)
+      echo "🚨 Unrecognized build '$BUILD'."
 
       if [ -z "$DEBUG" ]; then
-        continue
+        exit 1
       else
-        echo "⚠️ Would skip this, but DEBUG is enabled."
+        echo "⚠️ Would exit here with code '1', but DEBUG is enabled."
       fi
-    fi
-  fi
-
-  for BUILD in "${BUILDS[@]}"; do
-    echo "🛠 Building '$BUILD' from '$DOCKERFILE'"
-    case $BUILD in
-      release)
-        # build the latest release
-        # shellcheck disable=SC2068
-        ./build-latest.sh $@ || ERROR=1
-        ;;
-      prerelease)
-        # build the latest pre-release
-        # shellcheck disable=SC2068
-        PRERELEASE=true ./build-latest.sh $@ || ERROR=1
-        ;;
-      branches)
-        # build all branches
-        # shellcheck disable=SC2068
-        ./build-branches.sh $@ || ERROR=1
-        ;;
-      special)
-        # special build
-        # shellcheck disable=SC2068
-        #SRC_ORG=lampwins TAG=webhooks-backend ./build.sh "feature/webhooks-backend" $@ || ERROR=1
-        echo "✅ No special builds today."
-        ;;
-      *)
-        echo "🚨 Unrecognized build '$BUILD'."
-
-        if [ -z "$DEBUG" ]; then
-          exit 1
-        else
-          echo "⚠️ Would exit here with code '1', but DEBUG is enabled."
-        fi
-        ;;
-    esac
-  done
+      ;;
+  esac
 done
 
 exit $ERROR