Add a devcontainer for development

.devcontainer/Dockerfile

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+FROM golang:1.22-bullseye
+
+# Creates an app directory to hold your app’s source code
+WORKDIR /app

.devcontainer/devcontainer.json

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+{
+    "name": "Go PDF Sign",
+    "dockerFile": "Dockerfile",
+    "runArgs": ["--cap-add=SYS_PTRACE", "--security-opt", "seccomp=unconfined"],
+    "customizations": {
+        "vscode": {
+            "settings": {
+                "terminal.integrated.shell.linux": "/bin/bash",
+                "go.toolsManagement.checkForUpdates": "local",
+                "go.useLanguageServer": true,
+                "go.gopath": "/go"
+            },
+            "extensions": ["golang.go"]
+        }
+    },
+    "forwardPorts": [],
+    "postCreateCommand": "go mod download",
+    "remoteUser": "root"
+}