fix: restore mysql docker build

docker/tinode/Dockerfile.build

@@ -33,14 +33,10 @@ ENV NO_DB_INIT=true
 ENV SAMPLE_DATA=
 ENV DEFAULT_COUNTRY_CODE=US
 
-# DB connection is provided at deploy time via secret-backed env var. No default value.
-ENV MYSQL_DSN=
 ENV STORE_USE_ADAPTER=mysql
 ENV REDIS_ADDR=
 ENV REDIS_USERNAME=
-ENV REDIS_PASSWORD=
 ENV REDIS_DB=0
-ENV REDIS_KEY_PREFIX=im
 ENV REDIS_DIAL_TIMEOUT_MS=2000
 ENV REDIS_READ_TIMEOUT_MS=2000
 ENV REDIS_WRITE_TIMEOUT_MS=2000
@@ -72,13 +68,6 @@ ENV SERVER_STATUS_PATH=''
 ENV ACC_GC_ENABLED=false
 ENV PLUGIN_PYTHON_CHAT_BOT_ENABLED=false
 
-# Secrets: intentionally NO default values. Must be injected at deploy time.
-# An empty/missing value fails loudly at startup rather than silently falling
-# back to Tinode's publicly-known demo keys.
-ENV API_KEY_SALT=
-ENV AUTH_TOKEN_KEY=
-ENV UID_ENCRYPTION_KEY=
-
 RUN apk add --no-cache ca-certificates bash grep
 
 WORKDIR /opt/tinode