Fix shellcheck on remote containers (#20)

The shellcheck extension needs shellcheck to already be installed [1][2]

[1] https://github.com/timonwong/vscode-shellcheck#requirements
[2] https://github.com/koalaman/shellcheck#installing

.devcontainer/Dockerfile

@@ -5,12 +5,29 @@
 ARG VARIANT=buster
 FROM mcr.microsoft.com/vscode/devcontainers/base:${VARIANT}
 
+
+ARG GIT_EDITOR_SCRIPT_SOURCE="https://raw.githubusercontent.com/microsoft/vscode-dev-containers/master/containers/codespaces-linux/.devcontainer/git-ed.sh"
+ARG SHELLCHECK_VERSION="v0.7.1"
+SHELL ["/bin/bash", "-o", "pipefail", "-c"]
+# hadolint ignore=DL3008
+RUN apt-get update \
+    && export DEBIAN_FRONTEND=noninteractive \
+# Install xz-utils to extract tarballs
+    && apt-get -y install --no-install-recommends xz-utils \
+# Install shellcheck (apt-get doesn't install the latest version needed by VS code, so installing from GitHub release, see:
+# https://askubuntu.com/a/1228181)
+    && mkdir -p /tmp/shellcheck \
+    && wget -qO- "https://github.com/koalaman/shellcheck/releases/download/${SHELLCHECK_VERSION}/shellcheck-${SHELLCHECK_VERSION}.linux.x86_64.tar.xz" | tar -xJv -C /tmp/shellcheck/ \
+    && install -m 755 /tmp/shellcheck/shellcheck-${SHELLCHECK_VERSION}/shellcheck /usr/local/bin/shellcheck \
 # Configure VSCode as the default editor for git commits
 # Adapted from https://github.com/microsoft/vscode-dev-containers/blob/master/containers/codespaces-linux/.devcontainer/Dockerfile
-ARG GIT_EDITOR_SCRIPT_SOURCE="https://raw.githubusercontent.com/microsoft/vscode-dev-containers/master/containers/codespaces-linux/.devcontainer/git-ed.sh"
-RUN mkdir -p /tmp/git-ed \
-    && curl -sSL  ${GIT_EDITOR_SCRIPT_SOURCE} -o /tmp/git-ed/git-ed.sh \
+    && mkdir -p /tmp/git-ed \
+    && wget -qO /tmp/git-ed/git-ed.sh ${GIT_EDITOR_SCRIPT_SOURCE} \
     && install -m 755 /tmp/git-ed/git-ed.sh /usr/local/bin/git-ed.sh \
     && git config --global core.editor "/usr/local/bin/git-ed.sh" \
 # Clean up
+    && apt-get autoremove -y \
+    && apt-get clean -y \
+    && rm -rf /var/lib/apt/lists/* \
+    && rm -rf /tmp/shellcheck \
     && rm -rf /tmp/git-ed