read-file-to-env — Read files into environment variables and execute command

Example use:

read-file-to-env -one-line=HOST=/etc/hostname sh -c 'echo host=$HOST'

Use with systemd credentials, useful for example when a container image can only receive things via environment variables.
For example, postgres:14 (see https://github.com/docker-library/docs/blob/master/postgres/README.md for background).
For container images you control, please support reading from files.

[Service]
# only relevant lines shown, see `podman generate systemd`
LoadCredential=pg-user:/etc/keys/postgres-admin-username
LoadCredential=pg-password:/etc/keys/postgres-admin-password
ExecStart=read-file-to-env \
    -one-line=POSTGRES_USER=${CREDENTIALS_DIRECTORY}/pg-user \ 
    -one-line=POSTGRES_PASSWORD=${CREDENTIALS_DIRECTORY}/pg-password \ 
    -- \
    podman run \
    ... \
    --env=POSTGRES_USER \
    --env=POSTGRES_PASSWORD \
    postgres:14

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