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-branches.sh

@@ -3,6 +3,10 @@
 
 echo "▶️ $0 $*"
 
+###
+# Checking for the presence of GITHUB_OAUTH_CLIENT_ID
+# and GITHUB_OAUTH_CLIENT_SECRET
+###
 if [ -n "${GITHUB_OAUTH_CLIENT_ID}" ] && [ -n "${GITHUB_OAUTH_CLIENT_SECRET}" ]; then
   echo "🗝 Performing authenticated Github API calls."
   GITHUB_OAUTH_PARAMS="client_id=${GITHUB_OAUTH_CLIENT_ID}&client_secret=${GITHUB_OAUTH_CLIENT_SECRET}"
@@ -11,18 +15,33 @@ else
   GITHUB_OAUTH_PARAMS=""
 fi
 
+###
+# Calling Github to get the all branches
+###
 ORIGINAL_GITHUB_REPO="${SRC_ORG-netbox-community}/${SRC_REPO-netbox}"
 GITHUB_REPO="${GITHUB_REPO-$ORIGINAL_GITHUB_REPO}"
 URL_RELEASES="https://api.github.com/repos/${GITHUB_REPO}/branches?${GITHUB_OAUTH_PARAMS}"
 
+# Composing the JQ commans to extract the most recent version number
+JQ_BRANCHES='map(.name) | .[] | scan("^[^v].+") | match("^(master|develop).*") | .string'
+
 CURL="curl -sS"
 
-BRANCHES=$($CURL "${URL_RELEASES}" | jq -r 'map(.name) | .[] | scan("^[^v].+") | match("^(master|develop).*") | .string')
+# Querying the Github API to fetch all branches
+BRANCHES=$($CURL "${URL_RELEASES}" | jq -r "$JQ_BRANCHES")
 
+###
+# Building each branch
+###
+
+# keeping track whether an error occured
 ERROR=0
 
+# calling build.sh for each branch
 for BRANCH in $BRANCHES; do
   # shellcheck disable=SC2068
   ./build.sh "${BRANCH}" $@ || ERROR=1
 done
+
+# returning whether an error occured
 exit $ERROR