added gem feature

Author: patrickcrocker

src/gem/devcontainer-feature.json

@@ -0,0 +1,25 @@
+{
+    "name": "RubyGems",
+    "id": "gem",
+    "version": "1.0.0",
+    "description": "Installs RubyGems using the gem cli",
+    "options": {
+        "gem": {
+            "type": "string",
+            "description": "Name of gem(s) to install. Can be a single gem, or space/comma separated list of gems"
+        },
+        "version": {
+            "type": "string",
+            "description": "Specify version of gem to install",
+            "default": ""
+        },
+        "prerelease": {
+            "type": "boolean",
+            "description": "Allow prerelease versions of a gem to be installed. (Only for listed gems)",
+            "default": "false"
+        }
+    },
+    "installsAfter": [
+        "ghcr.io/devcontainers/features/common-utils"
+    ]
+}

src/gem/install.sh

@@ -0,0 +1,27 @@
+#!/usr/bin/env bash
+
+set -eu
+
+: ${GEM:=}
+gems=${GEM//,/ }
+
+if [ -z "${gems}" ]; then
+    echo "No RubyGems specified. Skip installation..."
+    exit 0
+fi
+
+if [ "$(id -u)" -ne 0 ]; then
+    echo -e 'Script must be run as root. Use sudo, su, or add "USER root" to your Dockerfile before running this script.'
+    exit 1
+fi
+
+export DEBIAN_FRONTEND=noninteractive
+
+args=("$gems")
+[ -n "${VERSION:-}" ] && args+=(--version "${VERSION}")
+[ -n "${PRERELEASE:-}" ] && args+=(--prerelease)
+
+echo "Installing gems..."
+gem install "${args[@]}"
+
+echo "Done!"

test/gem/install-cf-uaac.sh

@@ -0,0 +1,10 @@
+#!/bin/bash
+
+set -e
+
+# Import 'check' command
+source dev-container-features-test-lib
+
+check "uaac" uaac version
+
+reportResults

test/gem/scenarios.json

@@ -0,0 +1,12 @@
+{
+    "install-cf-uaac": {
+        "image": "ruby",
+        "features": {
+            "gem": {
+                "gem": "cf-uaac",
+                "version": "4.14.0",
+                "prerelease": false
+            }
+        }
+    }
+}

test/gem/test.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+set -e
+
+# Import 'check' command
+source dev-container-features-test-lib
+
+# Feature-specific tests
+no_op() {
+    echo "gem tests use scenarios.json instead so that we can specify using the ruby image"
+}
+
+check "empty test" no_op
+
+reportResults