Implement shellcheck recommendations #19

Job	Run time
Lint / Validate devcontainer-feature.json files	2s
Test / Feature actionlint	52s
Test / Feature actionlint » debian:latest	13s
Lint / Lint shell scripts	16s
Test / Feature postgresql-client	25s
Test / Feature actionlint » ubuntu:latest	17s
Test / Feature actionlint » mcr.microsoft.com/devcontainers/base:ubuntu	24s
Test / Feature shellcheck	38s
Test / Feature postgresql-client » debian:latest	16s
Test / Feature postgresql-client » ubuntu:latest	13s
Test / Feature postgresql-client » mcr.microsoft.com/devcontainers/base:ubuntu	30s
Test / Feature shellcheck » debian:latest	39s
Test / Feature shellcheck » ubuntu:latest	18s
Test / Feature shellcheck » mcr.microsoft.com/devcontainers/base:ubuntu	25s
Publish to GitHub Packages	8s
	5m 36s

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