feat: build-docker.sh: skip bitcoin-only variants for targets that do not have one

diff --git a/build-docker.sh b/build-docker.sh
index 45a34018..c2f001f4 100755
--- a/build-docker.sh
+++ b/build-docker.sh
@@ -294,9 +294,20 @@ for TREZOR_MODEL in ${MODELS[@]}; do
 
     MAKE_TARGETS=""
     for TARGET in ${CORE_TARGETS[@]}; do
+      if [ "$BITCOIN_ONLY" = "1" ]; then
+        # Skip targets that have no bitcoin-only variant.
+        case "$TARGET" in boardloader|bootloader|secmon|prodtest)
+            continue
+            ;;
+        esac
+      fi
       MAKE_TARGETS="$MAKE_TARGETS build_$TARGET"
     done
 
+    if [ -z "$MAKE_TARGETS" ]; then
+      continue
+    fi
+
     SCRIPT_NAME=".build_core_${TREZOR_MODEL}_${BITCOIN_ONLY}.sh"
     cat <<EOF > "build/$SCRIPT_NAME"
       # DO NOT MODIFY!