Setup .devcontainer for a GitHub Codespace (#19)

* Create a Codespace image with required tools for ops work.
* Add docs for GPG signing in a Codespace.

.devcontainer/Dockerfile

@@ -0,0 +1,20 @@
+# The universal image contains many useful tools including Docker-in-Docker already set up.
+FROM mcr.microsoft.com/vscode/devcontainers/universal
+
+# Add Cloud Foundry cf cli
+# The package source is unstable (frequent 401s) so we install the package directly.
+RUN curl -L -o /tmp/cf8.deb https://github.com/cloudfoundry/cli/releases/download/v8.8.3/cf8-cli-installer_8.8.3_x86-64.deb \
+    && dpkg -i /tmp/cf8.deb \
+    && rm /tmp/cf8.deb
+
+# Install other helpful tools
+RUN apt-get update \
+    && apt-get install -y netcat \
+    && apt-get install -y postgresql-client
+
+# Install the cf-service-connect plugin to allow easy access to Postgres
+# See: https://github.com/cloud-gov/cf-service-connect
+# Note: versions > 1.1.3 require a newer version of glibc than we have (2.31).
+USER codespace
+RUN /usr/bin/cf install-plugin https://github.com/cloud-gov/cf-service-connect/releases/download/v1.1.3/cf-service-connect_linux_amd64 -f
+USER root

.devcontainer/devcontainer.json

@@ -0,0 +1,5 @@
+{
+    "build": {
+        "dockerfile": "Dockerfile"
+    }
+}

docs/localDevelopment.md

@@ -30,3 +30,18 @@ Clean up:
 docker volume rm $(docker volume ls -q) # Removes local docker volumes
 docker rm -f $(docker ps -aq) # Removes local docker containers
 ```
+
+### GitHub Codespace
+This repo is configured with a [.devcontainer](/.devcontainer) to allow you to do development
+and ops work from within a GitHub Codespace.
+
+#### Signing Commits
+GitHub will refuse to recognize any other gpg key than the one that _it_ wants to sign with inside of
+the Codespace, which it automatically sets up for you. At least, as far as I've been able to
+determine. If you want signed commits (i.e. if your branch protection rules require it), just
+click the checkbox to enable GPG signing in your GitHub Codespaces settings (the settings attached
+to your GitHub account) and `git` in the Codespace will handle signing commits for you
+automatically. It'll work out of the box and you don't need to do anything else to enable this.
+
+Presumably this is handled automatically within VSCode as well if you want to use its GUI git
+tools, but I haven't tested this.