Pin Docker digests (#362)

Co-authored-by: Renovate Bot <[email protected]>

Author: renovate[bot]

Dockerfile

@@ -1,4 +1,4 @@
-FROM sourcegraph/alpine:3.10
+FROM sourcegraph/alpine:3.10@sha256:4d05cd5669726fc38823e92320659a6d1ef7879e62268adec5df658a0bacf65c
 
 # needed for `src lsif upload` and `src actions exec`
 RUN apk add --no-cache git

docker/campaign-volume-workspace/Dockerfile

@@ -2,7 +2,7 @@
 # that we use to run curl, git, and unzip against a Docker volume when using
 # the volume workspace.
 
-FROM alpine:3.12.3
+FROM alpine:3.12.3@sha256:074d3636ebda6dd446d0d00304c4454f468237fdacf08fb0eeac90bdbfa1bac7
 
 # Note that we have to configure git's user.email and user.name settings to
 # avoid issues when committing changes. These values are not used when creating