chore(Docker): add comments to explain the building steps (#1638)

docker/database/Dockerfile

@@ -1,7 +1,9 @@
 FROM alpine:latest as builder
 
+# install bash
 RUN apk add --no-cache bash
 
+# copy the sources from the host machine
 COPY apps /azerothcore/apps
 COPY bin /azerothcore/bin
 COPY conf /azerothcore/conf
@@ -9,10 +11,14 @@ COPY data /azerothcore/data
 COPY deps /azerothcore/deps
 COPY acore.json /azerothcore/acore.json
 
+# run the AzerothCore database assembler
 RUN ./azerothcore/bin/acore-db-asm 1
 
 FROM mysql:5.7
 
+# copy files from the previous build stage - see: https://docs.docker.com/develop/develop-images/multistage-build/
 COPY --from=builder /azerothcore/env/dist/sql /sql
 
+# adding the "generate-databases.sh" to the directory "/docker-entrypoint-initdb.d"
+# because all scripts included in that directory will automatically be executed when the docker container starts
 ADD docker/database/generate-databases.sh /docker-entrypoint-initdb.d