Strip matching quotes from --env-file values

[veeceey]

- What I did

Fixes #3630

I ran into this exact issue at work -- we share a single .env file between our local dev setup (where we source it) and our Docker containers (via --env-file). Any value with spaces in it needs quotes for source to work, but Docker was treating the quotes as literal characters. Drove me crazy for a while before I realized what was happening.

This adds quote stripping for values in --env-file parsing, matching how shells handle sourced files and how docker compose already works (fixed in docker/compose#8388).

- How I did it

Added a trimQuotes() helper in pkg/kvfile/kvfile.go that strips the outermost matching quotes (single or double) from values. Only strips when first and last characters are the same quote type -- mismatched or missing quotes are left as-is.

- How to verify it

Run the unit tests:

go test ./pkg/kvfile/... -v

The new TestTrimQuotes covers all the edge cases (matched/mismatched/nested/empty/single-char) and TestParseFromReaderQuotedValues verifies end-to-end parsing with various quoting styles.

You can also test manually:

echo 'FOO="hello world"' > /tmp/test.env
echo "BAR='single quoted'" >> /tmp/test.env
echo "BAZ=no quotes" >> /tmp/test.env
docker run --env-file /tmp/test.env alpine env

Before this change, FOO would be "hello world" (with literal quotes). After, it's hello world.

- Description for the changelog

docker run --env-file now strips matching surrounding quotes from values, consistent with shell source behavior and docker compose.