Add ShellCheck feature

Author: jgarber623-cargosense

.shellcheckrc

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+# Enable all optional checks
+enable=all
+
+# Follow source statements even when the file is not specified as input
+external-sources=true

src/shellcheck/README.md

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+# shellcheck
+
+Install [ShellCheck](https://www.shellcheck.net), a static analysis tool for shell scripts.
+
+## Usage
+
+```json
+"features": {
+  "ghcr.io/CargoSense/devcontainer-features/shellcheck:1": {}
+}
+```
+
+## OS Support
+
+This Feature should work on recent versions of Debian/Ubuntu and Linux distributions using the [apt](https://wiki.debian.org/AptCLI) management tool.

src/shellcheck/devcontainer-feature.json

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+{
+    "name": "ShellCheck",
+    "id": "shellcheck",
+    "version": "1.0.0",
+    "description": "Install ShellCheck, a static analysis tool for shell scripts.",
+    "installsAfter": [
+      "ghcr.io/devcontainers/features/common-utils"
+    ]
+  }
+  

src/shellcheck/install.sh

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+#!/usr/bin/env sh
+
+set -e
+
+if [ "$(id -u)" -ne 0 ]; then
+  printf 'Script must be run as root. Use sudo, su, or add "USER root" to your Dockerfile before running this script.'
+  exit 1
+fi
+
+apt update --yes
+apt install --no-install-recommends --yes shellcheck

test/shellcheck/test.sh

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+#!/usr/bin/env bash
+
+# This test file will be executed against an auto-generated devcontainer.json that
+# includes the 'shellcheck' Feature with no options.
+#
+# For more information, see: https://github.com/devcontainers/cli/blob/main/docs/features/test.md
+#
+# Eg:
+# {
+#    "image": "<..some-base-image...>",
+#    "features": {
+#      "shellcheck": {}
+#    },
+#    "remoteUser": "root"
+# }
+#
+# Thus, the value of all options will fall back to the default value in the
+# Feature's 'devcontainer-feature.json'.
+#
+# These scripts are run as 'root' by default. Although that can be changed
+# with the '--remote-user' flag.
+#
+# This test can be run with the following command:
+#
+#    devcontainer features test    \
+#               --features shellcheck   \
+#               --remote-user root \
+#               --skip-scenarios   \
+#               --base-image mcr.microsoft.com/devcontainers/base:ubuntu \
+#               /path/to/this/repo
+
+set -e
+
+# Optional: Import test library bundled with the devcontainer CLI
+# See https://github.com/devcontainers/cli/blob/HEAD/docs/features/test.md#dev-container-features-test-lib
+# Provides the 'check' and 'reportResults' commands.
+source dev-container-features-test-lib
+
+# Feature-specific tests
+# The 'check' command comes from the dev-container-features-test-lib. Syntax is...
+# check <LABEL> <cmd> [args...]
+check "version" shellcheck --version
+check "which shellcheck" bash -c "which shellcheck | grep /usr/bin/shellcheck"
+
+# Report result
+# If any of the checks above exited with a non-zero exit code, the test will fail.
+reportResults