add devcontainer

.devcontainer/Dockerfile

@@ -0,0 +1,26 @@
+# See here for image contents: https://github.com/microsoft/vscode-dev-containers/tree/v0.154.0/containers/ruby/.devcontainer/base.Dockerfile
+
+# [Choice] Ruby version: 2, 2.7, 2.6, 2.5
+ARG VARIANT="2"
+FROM mcr.microsoft.com/vscode/devcontainers/ruby:0-${VARIANT}
+
+ENV DEBIAN_FRONTEND=noninteractive
+
+COPY Gemfile /tmp/Gemfile
+
+RUN \
+  apt-get update \
+  && \
+  apt-get -y install --no-install-recommends \
+    jq \
+    libjq-dev \
+    libonig-dev \
+    gnupg2 \
+  && \
+  sh -c "$(curl --location https://taskfile.dev/install.sh)" -- -d -b /usr/local/bin \
+  && \
+  curl -fsSL https://raw.githubusercontent.com/helm/helm/master/scripts/get-helm-3 | /bin/bash - \
+  && \
+  bundle config set system 'true' \
+  && bundle install --gemfile /tmp/Gemfile \
+  && rm /tmp/Gemfile