Provide more context to *_regression.sh output

The sha256:xxxxxxxxxx output was kinda decontextualized, this provides
some context.

Tempted to just output to /dev/null but the SHA may be a good clue when
you're not doing what you want. E.g.: testing the syntax but you forgot
to save the file, then you get the same SHA as the last execution.

tests/highlight_regression.sh

@@ -4,6 +4,7 @@ set -euo pipefail
 
 cd "$(dirname "${BASH_SOURCE[0]}")"
 
+echo -n "using docker image "
 docker build --quiet -f docker/bat-test.dockerfile -t bat-test ..
 
 vol1="$PWD"/source:/tests/source:ro

tests/theme_regression.sh

@@ -4,6 +4,7 @@ set -euo pipefail
 
 cd "$(dirname "${BASH_SOURCE[0]}")"
 
+echo -n "using docker image "
 docker build --quiet -f docker/bat-test.dockerfile -t bat-test ..
 
 vol1="$PWD"/source:/tests/source:ro