Initial test build. Looks like everything works

.dockerignore

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+*.swp
+.git
+.gitignore
+.gitlab-ci.yml
+README.md

.gitlab-ci.yml

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+#
+# This guy is awesome: https://dev.to/bzinoun/gitlab-ci-to-build-and-push-containers-to-registry-538a
+#
+
+image: morlay/buildx:607a2ce
+variables:
+  # DinD's vfs driver is pretty intensive; this is less so
+  DOCKER_DRIVER: overlay2
+  CI_HUB_USERNAME: rehashedsalt
+  CI_PROJECT_NAME: minecraft-forge
+stages:
+  - build
+#  - test
+  - push
+services:
+  - docker:dind
+before_script:
+  - echo -n "$CI_LOGIN_PASSWORD" | docker login -u "$CI_LOGIN_USERNAME" --password-stdin
+  - docker version
+  - docker info
+after_script:
+  - docker logout hub.docker.com
+
+# BUILD
+Build:
+  stage: build
+  script:
+    - docker pull $CI_HUB_USERNAME/$CI_PROJECT_NAME:latest || true
+    - docker buildx create --use
+    - >
+      docker buildx build
+      --pull
+      --platform linux/amd64
+      --cache-from $CI_HUB_USERNAME/$CI_PROJECT_NAME:latest
+      --tag $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+      --push
+      .
+    - docker images
+
+# PUSH
+Push_When_Tag:
+  stage: push
+  only:
+    - tags
+  script:
+    - docker pull $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+    - >
+      docker tag
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_REF_NAME
+    - >
+      docker tag
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:latest
+    - docker images
+    - docker push $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_REF_NAME
+    - docker push $CI_HUB_USERNAME/$CI_PROJECT_NAME:latest
+Push_Bleeding:
+  stage: push
+  script:
+    - docker pull $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+    - >
+      docker tag
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:$CI_COMMIT_SHORT_SHA
+      $CI_HUB_USERNAME/$CI_PROJECT_NAME:bleeding
+    - docker images
+    - docker push $CI_HUB_USERNAME/$CI_PROJECT_NAME:bleeding

Dockerfile

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+# The first stage just builds up the modpack
+FROM alpine:latest AS build
+
+# Args
+ARG MINECRAFT_VERSION="1.16.5"
+ARG FORGE_VERSION="36.1.4"
+
+# The actual setup work
+COPY start-server.sh /minecraft/start-server.sh
+WORKDIR /minecraft
+RUN apk add curl openjdk8-jre &&\
+	curl -L "https://files.minecraftforge.net/maven/net/minecraftforge/forge/${MINECRAFT_VERSION}-${FORGE_VERSION}/forge-${MINECRAFT_VERSION}-${FORGE_VERSION}-installer.jar" -o installer.jar &&\
+	java -jar installer.jar --installServer &&\
+	echo "eula=true" > eula.txt &&\
+	rm installer.jar installer.jar.log && \
+	ln -s "forge-${MINECRAFT_VERSION}-${FORGE_VERSION}.jar" server.jar
+
+# The second stage is the actual container
+FROM openjdk:8
+WORKDIR /minecraft
+COPY --from=build /minecraft .
+CMD [ "bash", "start-server.sh" ]
+EXPOSE 25565

README.md

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+# docker-minecraft
+
+A Dockerfile and some assorted scripts to build a Minecraft Forge server container.
+
+## Usage
+
+Deploy as expected:
+
+```bash
+docker run -p 25565:25565 rehashedsalt/minecraft-forge
+```
+
+And badda bing, badda boom, you'll have a Minecraft Forge server of whatever the latest stable is.
+
+## Application State
+
+For obvious reasons, you'll want to consult the documentation of any mods you have installed, but a majority of application state lives here-ish:
+
+* `/minecraft/world`
+
+* `/minecraft/ops.json`, `whitelist.json`, `banned-ips.json`, `banned-players.json`, etc.
+
+* `/minecraft/logs`
+
+## Environment Variables
+
+variable|description
+---|---
+`JRE_XMX`|Maximum amount of heap passed to the main Minecraft process
+`JRE_XMS`|Minimum heap size passed to the main Minecraft process
+`ARGS`|Any additional arguments to be passed to the JVM
+
+## Useful Arguments
+
+The following arguments may be useful to pass through `$ARGS` from time to time:
+
+* `-Dfml.queryResult=confirm` - Useful when confirming through world corruption
+
+## Weird Defaults
+
+The default `server.properties` contains the following changes to better facilitate usage in a modded environment:
+
+```
+allow-flight=true
+difficulty=hard
+enable-command-block=true
+spawn-protection=0
+```
+
+## License
+
+MIT

server.properties

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+#Minecraft server properties
+#Fri Apr 09 16:05:21 CDT 2021
+allow-flight=true
+allow-nether=true
+broadcast-console-to-ops=true
+broadcast-rcon-to-ops=true
+difficulty=hard
+enable-command-block=true
+enable-jmx-monitoring=false
+enable-query=false
+enable-rcon=false
+enable-status=true
+enforce-whitelist=false
+entity-broadcast-range-percentage=100
+force-gamemode=false
+function-permission-level=2
+gamemode=survival
+generate-structures=true
+generator-settings=
+hardcore=false
+level-name=world
+level-seed=
+level-type=default
+max-build-height=256
+max-players=20
+max-tick-time=60000
+max-world-size=29999984
+motd=A Minecraft Server
+network-compression-threshold=256
+online-mode=true
+op-permission-level=4
+player-idle-timeout=0
+prevent-proxy-connections=false
+pvp=true
+query.port=25565
+rate-limit=0
+rcon.password=
+rcon.port=25575
+resource-pack=
+resource-pack-sha1=
+server-ip=
+server-port=25565
+snooper-enabled=true
+spawn-animals=true
+spawn-monsters=true
+spawn-npcs=true
+spawn-protection=0
+sync-chunk-writes=true
+text-filtering-config=
+use-native-transport=true
+view-distance=10
+white-list=false

start-server.sh

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+#! /bin/sh
+#
+# start-server.sh
+# Copyright (C) 2021 Vintage Salt <rehashedsalt@cock.li>
+#
+# Distributed under terms of the MIT license.
+#
+
+# Entrypoint will be server.jar
+args="-jar server.jar nogui"
+
+# Memory configuration
+[ -n "$JRE_XMX" ]	&& args="-Xmx$JRE_XMX $args"
+[ -n "$JRE_XMS" ]	&& args="-Xms$JRE_XMS $args"
+[ -n "$ARGS" ]		&& args="$ARGS $args"
+
+# Debugging info
+java -version
+echo "Invoking java with args: $args"
+
+# Start 'er up
+java $args
+